Out of curiosity, where exactly did you find that PROTON_USE_WINED3D11=1 from?
I got the feeling that could be useful when running Unity-based games with Proton on a GPU with amdgpu graphics driver.
Found it in a thread about steamplay compatibility in the game's steam discussion forum.
https://steamcommunity.com/app/670160/discussions/0/1734336452595889103/
Hello @derram, please copy your system information from steam (Steam -> Help -> System Information) and put it in a gist, then include a link to the gist in this issue report. Also, without the workaround, please add PROTON_LOG=1 %command% to the game's launch options and drag and drop the generated $HOME/steam-$APPID.log into the comment box.
PROTON_LOG=1x1 2018-12PROTON_USE_WINED3D11=1x2 2018-12PROTON_USE_WINED3D11=1 %command%x1 2018-12
I put in a lil over an hour playing on my desktop without any issues but couldn't get it to launch on my laptop.
Adding PROTON_USE_WINED3D11=1 %command% to the launch options got it to load up fine.