https://bugs.winehq.org/show_bug.cgi?id=56059
--- Comment #1 from Forest forestix@nom.one --- Maybe significant:
Saving highlights as webm files used to work in Wine when I was playing Overwatch 1, roughly 4 years ago. I don't know whether it broke due to a change in the game code for Overwatch 2 or due to a change in Wine.
Saving in mp4 format fails in a different way, producing an error message in game: "SAVING ERROR An unknown error has occurred. (6-04)" (This is not new, though. AFAIK, mp4 format never worked in Wine, even for Overwatch 1.)