it's happening to me too. I've ran out of data the past two months only to realize Spotify was the culprit. Did a test, Spotify downloaded 20 mb of data in less than 5 minutes!! What is going on here? Do I have to switch to Apple Music??
I haven't used Spotify much on cellular data because it has always seemed to absolutely hop off my data, so I usually use offline with downloaded playlists. I decided to turn on cellular data there earlier and it used up 194mb in about 10 minutes and I was playing off a downloaded playlist too!? I subscribe to Spotify through Vodafone and wondered if this was something that was done to intentionally use up your data as it's been going on for as long as I can remember.
Also getting this issue ever since 6.8.0, came back from a trip with 16GB left out of 17GB for the last 10 days of my billing cycle. Spotify managed to use up 15GB within 7 days from just 1.5 hour commutes each day and almost capped me if I didn't get the notifications from my courier. All my songs were already available offline too and somehow it still used so much. We need a fix ASAP, I'm already cosidering hopping to Apple music since it comes free with my plan and now it sounds more appealling than ever.
I've had this issue since the day DAILY MIX was released, about four months ago. I installed 7.5 BETA last week and the problem appears to be finally resolved. I've used 250 MB of data in a week; I was using that much in 15 minutes, listening to previously synced music!