The Glorious Eggroll team (as predicted) were quick to find the issue with the latest issue with the Diablo 4 beta and GE-Proton and have released a special version of GE-Proton7-51, easily identifiable as GE-Proton7-51-diablo_4_beta, however, it comes with a warning:
this build is for people who keep using Proton builds inside Lutris instead of Wine-GE builds and/or people trying to run Diablo IV through battle.net directly on the Steam Deck without Lutris.
This release is ONLY for Diablo 4 and should -not- be needed in the future.
TLDR: Wine commit: GloriousEggroll/proton-wine@8c23e2e breaks D4. Reverting it fixed it.
I have no idea if this commit is needed for other games, so I advise NOT using this build for other games.
Hopefully, the next version of GE-Proton will have both fixes in place, but for now, if you want the newer proton you can update, or you can carry on using GE-Proton7-27 to play Diablo IV for now. Full GE-Proton7-51 Diablo 4 Beta patch notes here
If you don't know how to install and manage custom Proton versions, you can check out our video below