Compatibility tool versions tested: Proton 9.0 (stable), Proton Experimental — crash reproduces on both

System info:

GPU: AMD Radeon RX 7900 XTX
Mesa/RADV: 26.0.3
Kernel: 7.0.0
Distro: Kubuntu

Steps to reproduce:

Launch Monster Hunter Wilds
Let it load through to the server connection / DLC verification / autosave warning screen
Dismiss the autosave warning
Game crashes within a few seconds

Observed behavior:
Game shows a crash dialog: Failed to create SourceReader with registered byte stream handler. error(c00d36c4) (MF_E_UNSUPPORTED_BYTESTREAM_TYPE), followed by repeated EXCEPTION_ACCESS_VIOLATION (c0000005). Proton log shows err:dmo:create_placeholder_file Creating tag file h264-used immediately before the crash loop, suggesting the crash occurs while attempting H.264 video decode via winedmo.

Ruled out:

Not GPU/driver-related (reproduces across 5 different Proton builds)
Not memory/OOM related
Not mod-related (reproduces with all mods, including REFramework, removed)
Not save-data related (reproduces on fresh install with no save)
Setting WINEDLLOVERRIDES="winedmo=" does not resolve it — instead fails earlier with Library winedmo.dll (which is needed by "mfmp4srcsnk.dll") not found, then crashes the same way, indicating no fallback path exists for MP4 playback without winedmo
Host FFmpeg (8.0.1) has H.264 decode support confirmed present and registered

Proton log: steam-2246340.zip