Hmm. Plays fine for me in Firefox on the latest Debian, and a Fedora spin too.
Discussion
The problem is HEVC requires license fees and is not widely supported on older devices, so I would recommend against it for Nostr content
anyway I installed the codec and it works
Yeah it's not default in the new Amethyst, it's an option. But if the results of these comments are anything to go by, every single one other than yours is positive, and it works. It's a choice to be made. Less bandwidth/better quality/some can't view vs bigger file/less quality/a few more users can see it.