This is a known issue, and as soon as Kisak sees it, it'll get merged into the existing issue https://github.com/ValveSoftware/Proton/issues/175
Hello @NullOtoko, we're using one issue report per unofficially supported game title and this has been mentioned several times in #175. I'm going to point to https://github.com/ValveSoftware/Proton/issues/175#issuecomment-424128257 as the direct match.
Closing as a duplicate of https://github.com/ValveSoftware/Proton/issues/175#issuecomment-424128257.
Compatibility Report
System Information
I confirm:
steam-582010.log
Symptoms
Game crashes after final boss. Thread on issue can be found here.
https://www.reddit.com/r/MonsterHunter/comments/99cqi4/xeno_save_corruption_bug_does_not_exist_proof/
Reproduction
Just beat the final boss. It will happen.
I assume proton is missing the codecs listed here, as installing these is the fix on Windows.
https://www.microsoft.com/en-us/software-download/mediafeaturepack
https://www.microsoft.com/en-us/p/web-media-extensions/9n5tdp8vcmhs?activetab=pivot%3Aoverviewtab