Not a permanent solution but have been able to stop the looping sometimes by purging cache and editing the notification settings in the Plex app. This is not a fix as it only works some of the time and shouldn’t be necessary, but seems the issue has something to do with notification.
But clearly this is a major issue that needs a quick stable solution ASAP.
Edit:
Also try blocking notifications from Plex completely, that seems to at least stop the looping.
