yeah, I got the same issue: unreliable and downright weird readings on the storage graph. I first noticed it after migrating my old iphone x to a new mini 13, and at first it said there were a few GBs of apple music data... against maybe 245MB of actual music files... I've tried everything including uninstalling the music app to get rid of those GBs... nothing worked. After concentrating on the Apple Music storage, I then noticed the main storage bar was giving me random readings at random times (including a glorious moment in which I had 128GB free on a 128GB phone).
After that and some more random numbers I stopped caring (it still reads 4GBs of music data against 2GB of actual music files, though).