I have a similar problem, quite often all my sync'd items on my Android phone disappears.
This morning I started a sync of additional Music to my phone, but as I had to leave for office the sync did not finish. During commute I listen to synced music and when I arrive to office I got wifi again and the sync finished.
Now all my sync'd libraries and items have disappeared, I try search but nothing shows up.
(I have no problem connecting to the server and view and play items from there)
After I "swiped to kill" the Plex on my phone and start Plex again and I can see my sync'd items again.
Then my phone goes to sleep, after I while I wake it up again.
Pull up the Plex app, takes some time to load.....
Then "Recently added items" shows up on Plex/Home screen, with some of my recently synced albums.
Opening the "hamburger menu", none of my libraries are there.
I try to access an album from the "Recently added items" tiles, the album pages show up but cannot do anything.
Navigates back to the Plex/Home screen, which now is empty, and no sync'd libraries in the menu.
I am using the latest Plex App for Android. My device is a Xiaomi MI5 Pro with MIUI8 (based on Android 6.0.1).