I have all my music downloaded via WiFi, but spotify has somehow used 622mb of data in my current cycle, putting me well past my limit. The only time I use spotify on my phone is over Android Auto, playing the songs and lists that I downloaded via WiFi already. 

Project Fi was kind enough to refund those charges for now as they've had the issue too, but I doubt they'll do it forever. What's going on with this app on Android Auto? It does it when USB connected to my Mustang and it does it in phone Anddroid Auto mode while suction cupped to the windshield of my old corvette.

I turned off everything using cellular data in-app.