I have used apple music for about 2 months now and this is the second time this happened.
I would be busy playing music from a playlist and then switch to another playlist, but it would give a pop-up saying something like: "Song can't be played because you're not logged into apple music". I am logged ito apple music though. I then proceeded to quit iTunes and restart my device, it seems to be the only way to fix it.
When I launch apple music again, the whole playlist that was playing would be empty and all traces of the songs in that playlist would be gone, even if it appeared in other playlists.
It seems to remove those songs completely from my library.
This is really frustrating and I hope there is a solution.