Just updated my Spotify app (0.6.0.181), and found another bug.
Try this out: Make sure your waiting list is empty. Tap the bottom track of a playlist (the last one), shuffle off (& let it play) Add a track to the waiting list. Open the player (swipe up) and click/swipe to the next track. And whoops, player disappeared :')
Just encountered the same issue. Thanks for posting. Did you figure out how to get the player back though? I can't for the life of me...
EDIT: Okay, it looks like if I make a playlist with one song and let the song play through its entirety, the play bar comes back. I'd confirm this works if I wasn't so afraid of losing the bar and not being able to get it back--maybe someone else who has this issue can confirm? Hope this helps.