I have been experiencing the same exact issue on my Sprint network GSII. It'll be playing fine, and then the music will stop and all of a sudden all of my recently downloaded playlists will be gone. Sometimes when I go to redownload them they reappear immediately, as echoed by Timoteo. It's quite frustrating, as the App works well aside from this one issue. It happened to me twice today, as a matter of fact.
I also have an issue with the music not starting up again after a text message is received and a notification sound is played.
Otherwise, great update and looking forward to the actual release!