Hello @GamerTechUni, noted at the top of https://github.com/ValveSoftware/Proton/wiki/Changelog. The builds of DXVK and VKD3D-Proton that are part of Proton Experimental requires mesa 22.0 or newer.
The version of mesa you're using inside the Flatpak environment is too old and this minimum driver requirement is not a bug.
proton experimentalx1 2022-08
System Information
Symptoms
Any game that is run under proton crashes and does not work. I get 'failed to initialize graphics' or just a crash.
How to reproduce
Run any windows game under proton