All I’ve ever had to do was leave the beta program on my phone in the Playstore, then either wait a couple minutes or refresh the Playstore on the Shield by force stopping it then relaunching.
You may need to uninstall the Plex app on the Shield then reinstall it if it gets stuck.
All that happens when you do this is your pinned libraries are reset.
This is specifically talking about the Plex client app, not the server app on the Shield.
