Note: I tried various solutions that are written about here on StackOverflow (example here). Please do not close this without checking if your solution from what you\'ve fou
We Don't have anything to do.
Once your app is not whitelisted it will be always killed once removed from recent-apps.
Because Original Equipment Manufacturer (OMEs) constantly violating Android compliance.
So If your app is not whitelisted from the device Manufacture it won't fire any background work even alarms - in case your app is removed from recent-apps.
You can find a list of devices with that behavior here ALSO you might find a side-solution, However, it won't work well.