I had a similar issue. The iphone storage numbers were a bit wonky for the past few releases, but seemed to correct in 15.1 beta 2. on installing beta 2, it showed i had over 20gb of “system data.” Deleting the music app off the phone, restarting and then re downloading the app from the App Store fixed the problem. System data is now hovering around 6gb. I suppose this is easy if you have Apple Music and don’t need to worry about locally stored music.