I am having this same issue. Everytime I restart my phone I have to redownload the app and all my music. This is not what I am paying a subscription for. This might be the end of my premium subscription if it is not fixed soon.
Not happy, this bug has been going on for ages and is still the same.
I've got about 50 meg of data left for this month so even if this works I won't be able to listen to anything since all my downloads will be gone. For a second time this month since I have just had to do a similar solution to stop the app randomly deleting my downloaded playlists.
Spotify, when are you going to fix this problem? This should have been fixed long ago. I too am a premium customer who is growing increasingly frustrated with the your apparent indifference to what is a serious issue. Fix it!!