Show us the output plz:
inxi --admin --verbosity=5 --filter --no-host --width
Did you followed the steps in my guide?
Disable Fast Boot in Bios.
How to activate Steam Play and Proton?
To switch to Proprietary Driver for your nvidia card.
It all should make a difference… the question is, if you had your settings already done or not.
Did you try to start exact this game with the same Steam Proton settings under the non-flatpak steam install?