There are still a lot of issues with adaptation of HEVC, especially with chromium based browsers on linux. Probably due to the patent fees or… whatever.
forum.endeavouros.com/t/…/72984
github.com/brave/brave-browser/issues/40101
This isn’t just a Brave issue, but affects basically all Chromium forks other than Thorium and… the other one I forgot the name of lol.
I can’t play any of the example hevc encodes, regardless of used flags/arguments. Vaapi set up correctly, and working everywhere else.

BUT, for the one person on here with the same issue, using Jellyfin. Thorium browser gets updated once in a blue moon, but does have a workaround for HEVC decoding, and if you don’t wanna use that, Jellyfin Media Player seems to be using mpv as the player backend, doesn’t seem to be just an electron wrapper, and works great.
Furryfox works fine too.