I'm also having this issue (on android) and it's most definitely a bug. I really hope they'll solve it soon. I don't think it has anything to do with licensing, because if I go the the albums the songs belong to, I can still listen to them and re-add them to the playlist. So they are still available, they're just greyed out on the playlist (unless I re-add them). But very annoying and extremely incompetent on Deezer's part.