How do I prevent Music app from using 1 GB of cellular data in less than 10 minutes?
This was a bug in iOS 8; it was fixed in later versions of iOS.

I encountered it personally whilst I was working at Apple and so did others: https://twitter.com/BenedictEvans/status/519966240484622336

More information: https://discussions.apple.com/thread/4351602

Even doing everything to prevent it: https://www.imore.com/apple-music-eating-your-iphones-cellular-data-plan-heres-how-fix-it the problem still occurred.