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Iphone X 11.4.1
I have the exact same problem
It SEEMED to be working, at it worked all right for about 2 days or so. After, that, slowly, slowly, songs have stated not being available offine, same as before.
Amazing this is a reported problem since a couple of years ago and Spotify could not care less...
Maybe only if all of us who are Premium users start unsuscribing they will think about improving their service. Also, someone from Spotify could say something in this thread...