If I disable the Online Media Sources they disappear from all my clients, including my Shield (2015 Pro). If Live TV is pinned at the time, it does not appear the next time I launch the Plex app.
When the Online Media Sources are enabled, Plex respects my pinned/unpinned choices on my Shield. I’m running version 8.11.0.21963 (beta). I did not see anything applicable in the beta release notes, but possibly behavior has changed between 8.9.x and 8.11.x.
I normally keep the online media sources disabled, so cannot say how earlier versions of the Plex app behaved on my Shield.
Only suggestions I can think of are the rather generic items of resetting the customizations and clearing any cached data. They won’t help if what you are experiencing is a bug. They might help if the app is just in a strange state for some reason.
Reset Customizations - This resets what libraries are pinned. Select the three dots in the upper right corner of the Plex app home page. You’ll see the option to reset customizations.
Clear Cached Data - Sign out of the app, then go into the Shield settings, force stop the app then clear data. Re-launch the Plex app, sign in, etc.