Updates remove saved music? Why?!?!

[Kirshy]

Quite frequently when both my Android phone and ipad updates, all my saved (downloaded) music will 'unsave'. I've had to redownload all my music at least 6-8 times this year! Is this a problem on my end or is it an issue with the update process itself? Seems to be the update process as it does it on two devices which are on different platforms. It's quite annoying as I fly frequently,  and I go to listen to some music offline, and there is none there!