Don't have a clue, but I don't care for its persistence. The solution I've come to is disabling Play Music entirely, loaded my entire MP3 library onto a Samsung 32 GB microSD card, and I listen to my music through PlayerPro. It is a paid app (less than $5) but it has a terrific UI, wide range of options, lock screen widget, expandable notifications, and is very easy on battery life. You just have to make sure immediately after installing it that you go into the settings and de-select the "automatically download" album art option or else it'll start doing that in the background.