I too have had Apple Music heavily discharging my battery on my iPhone 7 (as I previously reported). I deleted Apple Music (despite having a subscription). The battery discharge rate, during idle times such as overnight, changed to normal [expectations] according to the histogram in Battery setting. It is now a deletable Apple application.I see Apple Music as active in Background Activity. How do I turn it off? I don't use Music on my phone (iphone SE).
A few days later I reloaded the Apple Music app from the App Store. My battery still shows normal discharge behavior with the newly loaded Music app—with no background activity during idle times for the phone.
So one cause for fast battery discharge seems to be the Music app as delivered in the 13.x updates.