The Nokia 5.1 seems to be awfully eager to kill background apps. Apps like Whatsapp keep getting killed and there is no background activity manager setting at Settings -> Battery.
You should try disabling "Battery Optimization" for the apps which you want to keep running infinitely in the background.
The only solution I found to this problem was to use adb to blow away all the com.evenwell bloat that HMD seems insistent on loading. Why they felt they know how to do power management better than the standard Android settings (especially with doze) I have no idea. So far no ill effects from getting rid of all evenwell packages - just standard Android goodness.
Well, maybe they thought they will do that, because of the ridiculous batterys with lesser than 3000 mAh.
Yeah, the fact this doesn't really seem to bother them does not convince me to buy a Nokia phone next time or encourage others to do so. Really this seems a rather problematic issue, apps are restarting all the time which is really hurting multitasking on top of the fact I can permanently not use Twilight anymore for screen filtering.
Paul (if you read this), does taking out the HMD software affect battery life very badly? I'm not use to very impressive battery life, so if it lasts just one day thats fine with me.
@Eic that really sucks. I noticed that sometimes notifications, for example from WhatsApp, suddenly vanish again. When I then switch to the app it restarts. If nothing has happened in a year, then I have very little confidence this will get solved. I also contacted support to ask them to bring this issue to the attention of the developers.
I really don't want to go through the trouble of returning my phone but this is almost forcing me to. Alternatively I might root the phone and install a decent Android version.