Offline Apple Music removes itself.

Does anyone else have this problem?

I add playlists to my library, mark them for offline listening.

Two things then occur..

1. The music doesn't actually download locally (so I have to go and manually download each track in the playlist)
2. The music does download, but gets randomly removed after a couple of days. I've now got lots of empty playlists that are no longer functional for offline listening.

Everything works fine when on wifi - but not via 4G/3G.

Thanks,

Neil