Exactly the same problem here for me. If a sync stuffs on Monday for example, I can access them offline without any problem for the whole day.
But the Tuesday morning, when I open the app, the login screen appears, then disappears after like one second and I'm logged in without doing nothing. BUT all my synched playlists have been wiped and have been moved to the online only playlist.
I tried to 2 times to clear data, erase spotify folder, desinstall app, turn off the phone, remove battery, reinstall app without ANY improvement!!!
I suffer from this for the last two months I guess. No need to say it's really annoying and that it is definitely not what I'm paying for....