For AMOLED, TRUE EDGE, or TRUE AMPS to work as intented, please follow the instructions below.

Most Nokia phones (Power saver AKA com.evenwell.powersaving.g3)

1. Go to Phone settings > Apps > See all apps.

2. Tap on the right top corner menu > Show system.

3. Find Power saver app in the list, select it and Force close. It will remain stopped for a while, but will restart itself eventually.

From now on, background apps should work normally and use the standard Android battery optimizations.