Well just found a nasty bug. I added All Billy Joel albums to my library, a few hundred songs. I click Make Available Offline, downloaded them all. Was just showing off the feature to someone, click Make Available Offline again and it's now re-downloading all those tracks again. I can also confirm it deleted the tracks as my storage usage dropped drastically. They need to correct this or make Available Offline like Spotify which is a slider selection to turn it on and off for artists/albums/playlists.