I’m finding that when a show is about to end usually with in less than 4 minutes the audio kicks out to only the right channel. If I stop and restart it goes back to normal.
Possibly related to another issue where playback ends ~4 minutes before the video ends: https://forums.plex.tv/discussion/292602/video-stops-and-starts-4-5-times-last-1min-of-the-show#latest
Please try to disable Auto Adjust Quality in settings and see if that helps.
