I have tried the latest versions of Proton (experimental included) and the game didn't even start. For Proton versions [ Experimental, 9.0-2, 8.0-5 ] saw an error -> | Failed to initialize graphics. Make sure you have DirectX 11 installed, and have up to date drivers for your graphics card and have not disabled 3D acceleration in display settings. InitializeEngineGraphics failed |

For Proton version 7.0-6, I saw endless flickering, the game window was black and it was trying to resize itself to fullscreen, then it closed and again - endless cycle. It was doing that constantly until I managed to close it from the Steam window using the 'stop' button from the library section of the game.

Only with Proton version 6.3-8, the game worked without any problem. I haven't tried any other version after that.