SOLVED: Unstable: Firefox/Chromium not playing AV1

[Kiwa]

Cannot reproduce. From your ffmpeg log the browser aborts playing before the server remux finished, which means it is either the web player is not happy with your video or something else happened.

I'm going to convert another video to AV1 with other parameters and check if this is not the cause, but it doesn't make sense because it doesn't give any problems with Jellyfin Media Player.