Also tried using options PROTON_NO_FSYNC=1 PROTON_NO_ESYNC=1 %command%, no changes.
Temporary solution found for both the Steam Deck and my Desktop PC.
"/home/deck/.local/share/Steam/steamapps/common/Lost Ember"LostEmber/Content/Movies/rm -rf Mooneye_Reveal_Logo_1920.mp4 MooneyeLogoAnimFrom there, the game works perfectly (well, at first run there's a tiny screen on the desktop, but it's a minor issue). This solution also means that the reason the game was not starting was due to the starting movies. Codec issue? mfplat?
Compatibility Report
System Information
I confirm:
Symptoms
Reproduction
Same as symptoms
steam-563840-steam-deck.log
steam-563840-5900x-5x600xt-proton703.log