This can be fixed with -force-d3d9 as a launch option, i'm not certain if this is also a windows issue.
Hello @oct2pus, we'll be using one issue report to track each unofficially supported game title, so I've gone ahead and transferred this to https://github.com/ValveSoftware/Proton/issues/23#issuecomment-415536345.
I am running Debian Buster, and with the steam beta version of Proton I have a variety of issues compared to running Steam in Wine Staging.
I had set the game to use Fullscreen, and now with Fullscreen, the game stalls until Gnome prompts me to kill the process.
Edit: I have a Radeon RX 570, and I am running Gnome under Wayland (So Steam is running under xWayland)