QUICK FIX
Physically unplugging my Apple TV 4 and plugging it back in fixed this issue. Simply restarting it from the settings menu did not fix the issue, I tried that 3-4 times while physically unplugging the Apple TV relieved the issue on the first try.
The bug led to a few issues:
- first noticed it while playing a video in Plex and flicking trackpad up to reveal the time/progression bar, the time bar displays for a moment then disappears. unable to use the trackpad to display the time bar (swipe up) or the info/chapters bar (swipe down) from that point on
- other buttons did not work either (I forgot to try volume while I was experiecing the bug, but menu and TV button did not do anything)
- holding the menu button, however, did function properly, it returned me to the main tvOS menu except I could still hear the audio from the video Plex was playing as if it was still on, even while entering other apps
- re-entering Plex at this point and playing a new movie would play the new movie with the same bug, removing the lingering sound from the original video
- tried Netflix app while this bug was active and the trackpad up/down menus and other buttons worked no problem, so this was definitely a Plex-specific issue (playing the video also stopped the lingering audio from the Plex bug)
- as I wrote above, restarting the Apple TV by physically unplugging the power relieved the issue while the restart function in settings menu did not
- another interesting tidbit that might help diagnose the issue was that entering the Plex app after restarting in settings menu always caused the app to open on the last played video screen (new screen that was added fairly recently, displays after a video has finished playing, it has a thumbnail of the finished video and allows the user to replay it while showing a few recommendations at the bottom), this last played video screen was for the same video I was playing last night, my Apple TV would have been on this screen when it entered sleep mode due to being idle (as I went to sleep myself)