Why did Apple Music eat all of my data when I wasn't using it?

Yesterday afternoon while I was out shopping I got an text message from my network provider warning me that I had used 80% of my monthly data. I thought that was a bit odd as it renews on the 15th of the month so I shouldn’t have got through that much, but as I was busy I didn’t think too much more about it and just put my phone back in my pocket.

About 45 minutes later I got a text telling me that I had used up all of my monthly data. Say what? I hadn’t even used my phone at all in this time. I did notice the phone was pretty warm though, as if it had been busy doing something.

I checked the usage stats and it seems that the Music app had chomped through nearly 2GB of data in a short space of time. I hadn’t even been playing any music over the weekend, and there definitely wasn’t any music playing on my phone. It also doesn’t seem to have downloaded any additional music to my phone - the list of albums in “Downloaded Music” seems to be the same as it was before.

I’ve switched off mobile data for Music in the Settings menu for now, but apart from that is there any way to work out why it’s done this and how to stop it happening again? I recently updated to iOS 12.2 - might that be the cause? Would it have re-downloaded all my downloaded music after it updated? If so that is pretty stupid.

It’s an iPhone 6, if that is relevant.

I know that Google Music has the option to download songs from the cloud to your device so they can be played when you don’t have a connection, so it’s possible that Apple Music has a similar feature that had been enabled somehow. That would consume a lot of data while not listening to anything.

As far as I can tell, these are all the Music settings that use data:

Automatic downloads
Cellular Data
Streaming
High Quality Streaming
Download (toggles music downloads over a cellular connection, which is somehow different than just turning off “Cellular Data” for the app)

Turn off one or all of these options and Music will never use data.

It must have been downloading a lot of stuff. Just streaming, even HQ streaming, wouldn’t have used 2GB of data in a few hours.