This is an obvious software error, it stopped working for me because of an update of the client on Android TV from 0.18.3 to 0.18.4 and it only does not work on Android TV.
I do not have any experimental features enabled. I got a suspicion that there is a problem with the codec on the Firestick. VLC player gives me only the audio playback. No video at all. Haven't found another videoplayer to test yet to confirm this but it looks like the problem lies more in the region of the Firestick then Jellyfin itself.