Running games via desktop works fine, they load into the game menus with audio.
Running the same games in gamemode, will display a continuous steam loading menu while ALSO playing the game menu audio, it just never loads the display.
I am currently using ULWGL-proton-8.0-5-3 but have also tried with proton experimental to the same results.
It is also not a specific game as i have done this same process with another bunch of games from various stores (gog, epic, amazon & custom games)
Here is the list i have tried so far with the same results:
Graveyard keeper (custom)
Control (epic)
Death Stranding (epic)
hell is other demons (epic)
star wars (epic)
oxygen not included (custom)
red dead redemption (custom)
sonic mania (epic)
tales of the neon sea (amazon)