Apple Music using huge amounts of data

My phone has been getting through huge amounts of data through the Apple Music app. I’ve been tracking usage and turning off things like background app refresh. This morning I had mobile data switched off and reset my current period usage. I started streaming a song over Wi-Fi at home then left for work. I listened to that one song and no other songs would play as mobile data was off.

I switched on mobile data to check my maps and listened to some previously downloaded music. When I got to work my music app data usage was showing as 1.8Gb in a half hour period.

How can this be possible? Could it be a SIM issue?