No custom ROMs, Fairphone 3. There was an already existing thread on the troubleshooting forum about my issue, this was the described solution (german):
Discussion
I use a custom ROM & probably (didn't check) disabled energy optimisation. Making backups since years with screen off & not giving a damn about how the backup goes.
I already restored random backups, which I did not check, twice on two different pleb phones - didn't have to watch restoration process, either - and all worked fine.
So, not sure, if this is a new issue or something, but when it comes to apps being frozen, shut down or whatever, it's pretty much always the OS' fault in 99.99% of cases.
Android, especially pleb ROMs, are extremely "optimising" the crap out of every app.
Samsung & Xiaomi pleb ROMs even kill system apps, if they want to.
I'm now wondering whether Fairphone does some crazy shit with the OS. Maybe there's a bug in it. I heard from others online, that Fairphone have a couple of bugs here & there, that no other phones ever experience.