So, I've got Spotify Premium on my S5, and I have redownloaded my library about 5 or 6 times just this month because it keeps deleting itself and I'm left with no offline music until I get to a place with stable enough internet that I can download music again. I have about 3000 songs, so redownloading is actually something I have to make time to do or else I don't get everything, and it's becoming really upsetting, because as much as I love Spotify and the app and the interface and everything, it's getting rather rediculous. I don't have any other devices on which I keep offline music, by the way, as I've seen that was an issue with some others. 

Please help 😞 I don't wanna go back to Play Music again