I have a system that seems to keep Live TV sessions open in activity log which causes the inability to open new remote live TV sessions. Restarting PMS resolves the issues. My gut tells me that closing the Apple TV app does not send the appropriate notice to the server to terminate.
I’m also seeing this issue, with the same server version (Version 1.21.1.3876) running on Ubuntu, and only when I’m using the Plex app on Apple TV.
When I run a ps aux | grep Plex, I don’t see any stuck Plex Tuner Service process, but am unable to tune an watch any channel requiring then a restart of the server.