I’ve found that as well with some files. At first I thought the issue was with the jellyfin instance had not enough capabilities, but after trying the same file with its subtitles, it worked on any other client: flatpak, web, and regular android. So for me it is the chromecast/androidtv device, so I am now looking what to replace it with

You know landlines are still in use, right?

So you "self"host elsewhere?