Are you trying to play 4k content on a non-4k tv and making the chromecast rescale content? That will result in these issues because the chromecast is a slow POS that needs to play 4k content on a 4k tv
Are you sure? The jellyfin client (web or app) knows what resolution it needs, and the transcode logs on my jellyfin server show an output resolution being explicitly defined. Both the source and player are 1080p (no 4K on my server) so I can’t be sure though
no 4k content, no transcoding (disabled server side), plain ol' 264/265 files, no other devices have issues. worked fine for the first day or two. it's possible there was a system update but I don't know, latest ATV app.