I am having the same problem. I keep my music on my 32GB external SD memory card. The playlists will work for a short time and then I'll try to play one and all the music is deleted from the playlist. Occasionally some of the music will still be in the playlist but most of the time all the music is gone. Usually it's all the music from ALL THE PLAYLISTS too. I have to add music to the playlists again to use the playlist. I've tried deleting the playlists and recreating them entirely. I've tried just adding music back to the lists. I've tried removing the SD card and cleaning the terminals. Nothing seems to help. I did notice on one occasion that my playlists were empty after the phone fell off my nightstand. That said, this problem appears to be random, I can not find any real pattern to when this will happen. The music files always remain on the card, I've never lost a file, it is only the playlist that is affected. After Googling this issue I'm surprised to see how many other people are dealing with this. With all the people writing apps for android devices I'm surprised nobody has a solution for this problem...yet.
Galaxy S3 model SCH-I535, Android Ver 4.1.1