https://bugs.winehq.org/show_bug.cgi?id=55596
--- Comment #16 from Rufusovis dmrcghn@gmail.com --- The reason for video playback not working is now known. Here is a workaround patch for it that applies to Proton: https://github.com/GloriousEggroll/proton-ge-custom/compare/master...Hawkhea...
In the words of the patch author, the behaviour is as follows: "It's trying to stuff a 1920x1080 video in a 1920x1088 buffer which gstreamer does not like" "it sets up to display a 1920x1088 video, tries to play back a 1080p video into it, and then asks MF 'so how big is the video anyways'"
Apparently Windows tolerates this but Wine doesn't.