Help! After signing up for Apple Music, 'Media Services' on iPhone is gobbling up all my cellular data despite my settings to not use it
After some extensive testing and looking at the data usage time period, I have concluded that I was bitten by the 'wifi assist' feature (which is now set to off).   The massive sync of songs to my device from the cloud was actually when I was or should have been connected to my wifi, and was 'assisted' by my cellular connection, even though that sync would not have been allowed given my settings if just on cellular and not wifi.