I can't reproduce this issue in my machine. This game works for me out-of-box.

System informations:

• GPU: Radeon RX 550
• Driver/LLVM version: 4.5 Mesa 19.3.3
• Kernel version: 5.4.0-3-amd64
• Link to full system information report as https://gist.github.com/jzengc/df7e554aa861661c7b1fda53e2ea102d:
• Proton version: 4.11-12

Also, this game is Platinum in ProtonDB: https://www.protondb.com/app/446120

Compatibility Report

• Name of the game with compatibility issues: Bunker Punks
• Steam AppID of the game: https://steamdb.info/app/446120/

System Information

• GPU: GTX 760
• Driver/LLVM version: NVIDIA 440.59
• Kernel version: 5.5.2
• https://github.com/shoober420/linux-scripts/blob/main/home/shoober420/sysinfonvidia
• Proton version: 4.11-12

LOG:
steam-446120.log

Symptoms

Same symptoms. It seems the whole GUI is bugged for me. When I go to options and switch resolutions from "Top Value" to 2048x1536, the drop down menu will stay open, even after clicking the desired resolution.

Another example is when clicking "Reset Game", whether or not I click "Yes Delete" or "No Thanks!", the window stays open, and it appears no changes are made.

The credits screen does not scroll like should.

Last major thing, is the previously mentioned issue with when starting a new game, the difficulty options, no matter if its "Easy" or "Brutal", will simply close the choose difficulty window, and go back to the main menu, with no new game starting.