My Spotify app used about 15 gb of data yesterday (all I had I left this month). I did not stream music nor had I the download om mobile network option turned on. I admit I have not read all the terms of agreement, but is this something Spotify are allowed to do? Does anybody have an idea of what caused this?
I once also had this problem, after uninstalling and reinstalling the spotify app, I didn't had any more problems. But if this doesn't work for you try to use your data saver in the settings of you phone. If all this doesn't help I would put your spotify app in the offline mode.