While I am not an Apple Music user, I have heard from a couple users that potentially the newly introduced Apple "lossless" quality streams may be not captured as unmetered on the Telstra/Boost network.
Maybe turn lossless off in the app settings and see how the data usage goes for a little while (as a test to see if that is the issue you are seeing).
If that does solve the issue, I would suggest letting Boost support know so they can follow it up for you.
* I am a Boost Australia customer. This is a peer to peer community where users help each other.