This day morning, I try to centralise my messed up iTunes library metadata, and sync them to my iPhone. After completing changing, I plug in my iPhone, and start syncing. It completes "as expected" with 97 items fail to sync. I don't care much, and move to music app. Guess what I find? None of my music is listed in music app, nor those in the cloud (purchased songs). After switching to mobile network, those purchased songs are back (not all of them because some of these are purchased in another store). And iTunes shows me 55 GB of "others", which is my entire music library, plus something else, about 3GB. Can't believe such massive metadata could break the entire library. Fortunately I completely disable Apple Music, or everything will be a mess. I use iOS 9 beta 5 on iPhone and iPad so I post it here. Plus I run the fifth beta of El Capitan.