Cancelling a knock-up with flash

In this clip (which is on 50% speed), Darius flashes during Rek'Sai's knock-up. Surely you shouldn't have control of your character while you are in air? He wasn't even coming back down yet, but rather still going up up and away. He didn't have cleanse or QSS or any other effect which would've returned the control. Is this a mechanic that has always been and/or is this working as intended?