Subtitles are the Achilles heel of Jellyfin (at least the Android client sending the stream to Chromecast stick connected to the TV). I can see the subtitle present but it doesn't show on the movie. Ok, I will just add manually - can't, progress indicator loops forever. Ok, I will remove the existing subtitles and add - removal succeeds - addition fails. This is the same fiddling everytime. If it works, it is a miracle. I used to forget about the subtitles and start the movie - you could just fix it on the fly right?! Nope, now I do full prep before calling everyone that the movie is good to go.

This is not coming from a bad place - I am very appreciative of having a open source alternative to Plex. Just putting this out there in case it catches someone's interest to improve it.

I had this problem, but I fixed it by setting it up so it extracts all embedded subtitles into SRT files, and enabled a setting prevents Jellyfin from serving embedded subtitles.

From what I remember, some clients don't support streaming subtitles, so they must download them completely in advance, when the movie starts, before they start appearing. If you pick embedded subtitles with these clients, Jellyfin must extract the subtitles files immediately, and this takes a while depending on container format, file sizes and server load. External subtitles are faster.

Is that an old problem, or a problem that is still in this new v12?

FWIW, Moonfin and Infuse both work flawlessly on an AppleTV.

I also remember Google TV working fine, but it seemed like ads got added to another part of the UI every month so I got sick of it.

Roku only supports text subtitles, so if you have picture subtitles (like a DVD or bluray), then you'll need to burn them in if you want to play on Roku.