I was able to try two things:
- Charging another GOS phone (Pixel 6a). It charges up to 100% without any issue.
- Charging my phone (the P7) when it was turned off. To my suprise, it did also charge to 100%.
Don't really know what that should tell me except that GOS is not the problem.
At least I can tell everyone who has made it til here and wants to buy a new powerbank, this model doesn't seem to be causing problems in general.