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Android: Screen off - notifications delayed / blocking

This issue I have may be only present on devices running Oreo or later as I have only a OnePlus 3T with me for development.
I have a foreground Service that is started using ContextCompat#startForegroundService() and in the service's startCommand method I make sure to create / enable my notification channel so that I can call startForeground with the associated on-going notification.
So I believe, in terms of requirements I get everything right here.
What does my Foreground Service do:
It acts as a Bluetooth Server using the Bluetooth Class RFCOMM BSPP to listen for client to connect ;
When a client connects, it listens for incoming requests ;
It performs the requests and send data accordingly.
My problem lies with the last point. When I connect and ask to send data, two things happen on the phone :
The relevant data is being loaded from the phone and
Once fully loaded, the data is sent to the client.
Each of these operations have their own notifications to display progress to the end-user.
While the first operation is not delayed, the second one may take many minutes (up to 6 in one of my tests).
After further tests, the cause is the notifications because I cancel the first operation notification then I call the second operation, which in turns will display and update its own notification.
So in the end, because I call NotificationManagerCompat.cancel on the first operation notification right before calling the second operation, that call can be delayed for a lot of time. If I don't cancel the notification first, then the second operation is immediately started.
As a solution, I believed that the Doze mode is responsible for that behavior, so I manually white-listed my app the battery optimisation settings of my phone. That did not change a single thing.
Finally, if, at any point, I unlock my phone / turn the screen on, the blocking / delaying notifications get's unlocked immediately and the second process starts right on the bat. So, of course, this behavior does not occur when my phone is plugged into my computer even with screen off.
Is this a bug ? Is this related to doze mode ? I do not know.
[Edit 1]
For better clarity, here is a code snippet that represents the sequence of operations:
loadData() {
... // defer heavy work on a worker thread and updates notification of progress from time to time (every 10%)
notifyDataLoadingStarted(); // on main thread. Displays its notification with progress bar
}
// once the data is fully loaded
onDataLoaded() { // called on the main thread
cancelDataLoadNotification(); // asks the notification manager compat to cancel notification
initiateDataSendingOperation(); // will also defer heavy work on a worker thread and update its notification accordingly
}
What happens is that onDataLoaded is called once the first operation is over. But the problem is that cancelDataLoadNotification is blocking. I am on the main thread and this methods returns very very lately. Hence, this is the reason why initiateDataSendingOperation is greatly delayed.
When I unlock my phone, I could see that the notification of the loading operation is stuck in the middle, as though the loading operation is halted, around 50%. However, this only lasts a fraction of a second because before I know it, the first notification pops out while the the second notification pops in. All in the blink of an eye. Which clearly means that yes, the first operation was indeed over since a long time ago, but its notification was definitely obsolete. And since the second operation would only be initiated when the first notification is cancelled, they you can clearly understand that the second operation is greatly delayed.
That's why my first workaround works if I first start the second operation before canceling the first operation notification.
Nevertheless, in any case, the notifications are still out of date while screen is off.

setExactAndAllowOnIdle doesnt trigger right

I have a problem using setExactAndAllowOnIdle. In my AlarmReceiver class I just show simple notification that shows when alarm was triggered and set another, same alarm but hour after.(currentTimeMillis + 60*60*1000). When my phone is in use application works fine, alarms come exactly on time. But when I let it work for few alarms without waking device up, they start to trigger with few minutes delays, or sometimes even exactly on time I wake up my phone.

You probably mean setExactAndAllowWhileIdle().
You didn't tell on which OS are You testing it but probably it's because of Doze Mode.
NOTICE:
Neither setAndAllowWhileIdle() nor setExactAndAllowWhileIdle() can
fire alarms more than once per 9 minutes, per app.
So You can't use this method to set every alarm what probably You doing.
For more information You can go here:
https://developer.android.com/training/monitoring-device-state/doze-standby.html

Defer timer until SCREEN ON event?

I have a service that runs periodically using a timer to invoke itself, but should not run when the screen is off. When the screen on event is fired, the service should run, but only if it's past when the timer would have fired.
Right now I still run the timer continually, but have the service do nothing if the screen is off. I can also run the service via a broadcast receiver when the screen turns on - but this runs the service every time the screen is turned on, instead of only when it's past when the timer should have run. Recording this state in the service doesn't seem to work as Android will kill the JVM for the app in between executions.
What would be the cleanest/correct way to implement this type of behavior?

So there are a couple of intents that you can listen for, Intent.ACTION_SCREEN_OFF and Intent.ACTION_SCREEN_ON. However, these don't work as manifest receivers, they need to be explicitly registered.
See this answer: https://stackoverflow.com/a/9478013/1306452
In all scenarios, it's going to involve a long running service in order to listen for those events.
One thing to keep in mind is that these intents only listen for when the device becomes non-interactive and vice versa, not specifically when the screen goes off (see the description on the Intents).
The best way for you to achieve this behaviour would be to listen for when these intents with a long lived service, started with START_STICKY to help guarantee that the service is running. The service can register a receiver for the SCREEN_ON and OFF events, and when it gets these events either do nothing if the timer has not elapsed, or continue if it has.
This won't be nice on your battery life, and what ever you are doing it doesn't sound like it's going to be a pleasent user experience. You might also want to step back and see if there's another way around this obstacle (my 2 cents).

Stay awake/wakelock for a specific duration?

I need my android application to stay alive for a short specific duration.
I have a long series of updates, during which I keep the screen alive with wake looks. After that is done, there is an dialog informing of the success. Before this one is posted however I've released the wake look. Here I want the application to have a wakelock for 30s and then release it even if the user dos not click ok on the dialog. This is because I dont want to drain the mobile.
Is there an easy way to do this? That is, once I've reached a certain stage in my code I want to have a wakelock for a short duration?

For any long running tasks you should use a service, and for your purpose a ForegroundService.
This comes with the requirement to have a notification visible while your service is running. You can then post a different notification once the process is complete.
This will make your processing resilient to the user stand-by'ing the phone, rotating the phone or going to another app, all of which would interrupt processing if originating in an activity.
http://developer.android.com/reference/android/app/Service.html#startForeground(int, android.app.Notification)
For ease of use, I usually override IntentService which has the least amount of boiler code required to get going.

keep service alive in android

I have created an Android service which basically does the following:
schedule a task, using the ScheduledThreadPoolExecutor.schedule() method
when the schedule time is reached, the task is executed and a new schedule is started
Thus, the service continuously has a timer in place (except during execution of the task); sometimes 2 (independent) timers are in place, depending on the result of the task. The service should be running all the time and executes different tasks from time to time.
All seems to work ok (at least when running with DDMS), but after a while (something like an hour without connecting the device via DDMS) the timer tasks are not executed anymore. The only way to get it working again is either stopping and starting the service again or by connecting the device with DDMS (using Eclipse); waking up the device only does not trigger this.
It looks like, Android sets the service in a kind of sleep mode (service is still running when looking at the Active Services on the device).
My question is: how do I prevent this behavior and keep the service working all the time?
I did some research and I found something which theoretically could give a solution, by acquiring a (PARTIAL_WAKE_LOCK) WakeLock in the service, but as far as I understand, using this solution, I have to acquire the lock at the onStartService() method and release it at onDestroy(), meaning that I keep the lock during the lifetime of the service (which is forever); I suspect a battery drainage using this method.
Is there another way to accomplish the same?

I have solved the problem, thanks to the replies above.
I switched from the ScheduledPoolThreaExecutor() to the AlarmManager. Together with the RTC_WAKEUP type all triggers are handled now.
I did not use the IntentService, because my service has to do things in parallel based on alarms (and IntentService implements a queued solution). The original service, which was staying alive all the time in the original implementation, is now only created whenever an alarm is triggered.
I also have the impression (but that is only a gut feeling, not based on real measurements, that this new implementation (where the service is created when needed and is not alive all the time (waiting on timer evens), is even better for battery life of the device.

How often to you need to execute, and what do you mean by running all the time? I guess that you don't mean be executing all the time?
I have a service that does this, and it works quite well:
It schedules an alarm with the alarm manager. Acquires a wakelock when the alarm is triggered, performs some work, and then schedules a new alarm before releasing the wake lock.

Use IntentService for your service implementation and register pending intents with AlarmManager to trigger those intents on the time basis you need.

I have used wake locks in an app before and I did not need to release them in the onDestroy(),
I literally had the following and it worked perfectly:
onClockListener{
acquire wakelock
method call
}
otherOnClickListener{
release wakelock
other method call
}
Not sure if it will help much but it definitely wont help if I don't post anything :)