Something i tried a few moments ago:
I sideloaded the APK you said was confirmed as last working from the other thread (Is anybody working at Plex going to fix the Nvidia shield startup freezing but audio plays and then catches up fix - #205 by ChuckPa)
and it works. No playback problems at all. So this is not an issue on my side only. If it worked before, an update in the main app or one of it’s dependencies caused this to happen.