Update: In the meantime, it is no longer only a problem with biometric unlocking:
Moreover I often only get out of apps if I lock the screen once and then unlock it again (If I'm lucky, it works with biometrics, otherwise I have to enter the password every time for the workaround).
Sometimes when I lock the screen, I can still see the app semi-transparent on the lock screen.
Backbutton, change keyboard language (I use the standard Graphene keyboard), recent app view - all does not work - The device does not respond to any of the mentioned gestures.
After restarting everything works again, but as soon as I have switched 1-2 times to other user profiles, the behavior occurs again.
I only use the Owner Profile (in my other profiles - with the exception of the error with biometric unlocking - the described problems do not occur) to install apps, so the errors don't affect me as much - but if I only had this profile, I'd say the phone isn't really usable in that state.
Edit: I'm using a P8Pro.