This is what I highly recommend for people having this problem with their Android Plex clients. I have been using the Plex for Kodi on my Firestick and both Android boxes. It is working perfectly. Use the Kodi client instead of the Plex client. It looks and behaves almost identically…. The notable difference is the Plex client for Kodi actually works.
I am not suggesting to give up on the Plex Android client. But rather use the Kodi client till Plex fixes the problems. And that could be months down the road. It should have been fixed months ago already.
And it’s not hard to setup. Download Kodi to the Android device. Then download the Plex add on. Make sure to use the Official Plex for Kodi add-on. The current version as of writing is 0.3.3. You can even have Plex set to open up immediately after opening Kodi. It’s still Plex.
I used this site to help with setup. How To Install Plex Kodi Addon on Firestick/Fire TV, and Android TV