My Tucson does that with the auto-defrost. I think it just bases it on outside temp, though I'm not sure how it knows when to switch back to front vents vs defroster.
I've been looking everywhere, and it seems like this is the one question I can't answer with a Google search or the spec sheet. I'm hoping that perhaps someone knows the answer or one of you eagle-eyed livestream viewers has noticed: what type of climate control is on the Pro trim? For me...