I'm getting a weird UI effect when finishing a battle, as of the November 11th patch. To reproduce, choose Practice Mode, Vs Enemies, Easy Difficulty (though any difficulty has the same problem), win the level. The reward screen has a "Back to main menu" button that wasn't there previously, partially occupying the same space as the reward chest.
Upon claiming the reward, this is the effect:
If you hit the weird button after finishing all 20 levels, the game bugs out.