Same issue here. I'm using 6s+ 128gb and cannot sync music to play offline. I deleted spotify (with my 3000+ offline songs!!!!) and reinstalled it now it's downloading all the songs but I had to choose them all one by one. This is really ridiculus.
Sometimes sync progress bar is disappearing so i have to turn spotify off and on again. There is no stability on this app. If I will have to delete it and install it again I will end my subscription for sure.