This happened a few times recently...

Beta11. After going from playing just 1 or 2 tracks (not in repeat) to all-track shuffle (three presses of the bottom key) the busy timeglass symbol would pop up like normal, but the music would keep playing. Then, after a short wait (have 8400ish tracks) I could pause the player with the top button and finally the root playlist was queued and ready to go.

I will try to recreate it and get more info.