This is happening to me too. It's extremely frustrating, especially since you can't delete songs from your library without removing them from playlists too. I updated to the newest version of IOS in hopes it would help-- no luck. As others have said, if you are really really quick on adding a song after turning the setting off yet again it seems to work sometimes, but that's annoying, unreliable, and only lets you add one song at a time.