Solved the issue. In the end, it is not any bug in proton or otherwise. It is just a game quirk. If you start the game in SteamVR mode, it doesn't use VR. And even in its options, it doesn't show any toggle for you to en...
Got a few more details with further tests: _Instrumented Proton vrclient’s `load_compositor_texture_dxvk()` path. Overload submits D3D11 OpenVR textures successfully, and vrclient translates them to Vulkan images passed ...