Try downgrading the version of the Plex client on your shield. Plex introduced a lovely bug that causes this without proper bug testing and now a large number of uses are affected and posting on the forums to not much avail.
Check out this thread for more info on affected versions. Plex on Nvidia Shield 2019 Pro freezes after 11-13 minutes playback
v7.28.x seems to work cleanly.
v7.29 Some release in here has the bug avoid these.
v7.30.0.16082 (currently beta) Seems to be working without ANY issue.
