Apps don't show notifications until opened and can't get started by other applications

Dear Android Forums!

first: yes i used the search function although i couldn't find a thread which addresses my issue in general, also i tried a few of the fixes regarding the notifications although it didn't solve my problem.

Phone: Doogee S88 Plus
OS: Android 10

I received this phone a while back and i'm pretty happy about it, although i noticed a very strange behaviour regarding nearly all apps.
Apperantly, every app that isn't a foreground app, will get killed or at least will not send notifications until they are opened which, you sure can think, is damn annoying.
Also, apps won't start with Android Auto, except Google Apps like News, calendar, maps and such. Spotify, Audible and every "external" apps will refuse to start on android auto until i open them manually on the phone, then they will show up on my car as well.

i checked the installed software and noticed a preinstalled energy saving app (which is funny with an 10Ah battery but well ) called com.cydroid.softmanager
Thinking this could be an issue i uninstalled/deactivated the app via ADB, although it didn't get better in any form.

And yes, of course i checked the battery optimization which is turned off for the usual apps which send notifications like Signal, Threema, Outlook, Authenticator and such
As i use Signal as my SMS/MMS app, i can say that i will not receive SMS/MMS notifications as well, i will receive them if i use the standard SMS app Messages though.
Also notifications in general are activated.

Maybe it has something to do with Doze? I'm currently at my wit's end.

I already thought about rooting the device, although it doesnt seems like that there is any rom out there for it, it has the option for OEM Bootloader unlock in the dev options though, but to be honest it's been a long while since i rooted android devices except from Samsung.

Hopefully you guys can help me!

Kind regards

**edit**
also i think it has to do with the background apps in general since foreground apps like my VPN (Cyberghost) will keep running without issues

also i noticed if i restart the phone all the apps will start correctly in the background, so it seems, since i'll get the notifications immediately