I just received this response from Steam support. Luckily, I can still play the game on my phone.
Thanks for reaching out to Steam Support and I'm sorry to hear that you are experiencing an issue running WGT Golf using the Steam Play Proton feature for Linux.
Currently, WGT Golf is not supported with Steam Play, so you're likely to encounter difficulties with the title playing it this way.
Please submit all issues or bugs occurring when using this new feature, to the Valve - Proton Github bug database using a GitHub account. This database is regularly reviewed by the Valve development team.
In the meantime, the best source for further troubleshooting steps and suggestions can be found in the Steam for Linux community discussion to see if other users have found a way to play WGT Golf using Steam Play, as well as consulting our Proton - Quick-start Guide.
If you have any further questions, please let us know - we will be happy to assist you.