Whenever you try to play something that needs to be transcoded Jellyfin will save the ffmpeg log. That should hopefully give you the info you need.

If the error isnt clear, share your hardware info and transcoding settings.

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Plex's transcoding may be better, but jellyfin's will work fine once you get the settings dialed in. A bit more info would be helpful, mainly: What kind of jellyfin install (docker, native package manager, podman)? What video codec is the file? Are you using hardware transcoding, and if so, what hardware?

Edit; oops, I thought you talked about both Apple and Android, I'm so used to ATV being Android tv. I might leave the comment anyway if it helps someone else.

For Android tv, if using the official app, I would recommend downloading "Just (Video) Player" and then configuring the Jellyfin app to use external player. On my Shield the videos play much better without transcoding

What sort of video were you trying to play? Also you said you were running it through docker via CasaOS. Things to note, you need to pass your gpu into the container. Relatively simple. Then you got to make sure hardware acceleration is enabled. Make sure HDR to SDR tone mapping is correct.

I have Home Assistant + Music Assistant that integrates Jellyfin. Works well with our iPad's to Denon Heos and such but I haven't tested it on my Apple TV yet, it is collecting dust because it doesn't do Stremio without having to fudge around.

What does your transcoding configuration look like?