https://github.com/ValveSoftware/Proton/issues/2927 Could this be what you're running into? If it is, an error should appear in dmesg whenever you launch Borderlands 3.
Also, the mfplat-related workaround you suggested should not be mentioned here as it is of questionable legality.
That appears to have been it. I hadn't run across any information about checking the dmesg for errors before this, though I had stumbled across the boot option fix. I thought I had already implemented it, but when I went to verify, it turns out I hadn't. Thank you for the help.
PROTON_USE_WINED3D11=1x1 2020-03PROTON_USE_WINED3D11=1 %command%x1 2020-03mfplat.dllx1 2020-03
Compatibility Report
System Information
I confirm:
This setting created two logs in the steamapps/common/Borderlands 3 directory, "Borderlands3_d3d9.log" and "Borderlands3_d3d11.log". Both files were blank.
Symptoms
Game fails to launch. No error messages are reported.
Troubleshooting Steps
As per related issues with getting the game to launch, performed following steps, with no change in behavior:
installed
<Workaround removed by moderator>packagecopied mfplat.dll into Borderlands 3 program folder
added "PROTON_USE_WINED3D11=1 %command%" to launch options
Reproduction
Game fails to launch following fresh install, and after above steps are taken.