Fiora ult health popping when champion is not dead and she didn't hit 4 vitals

I was spectating my friends game, and on his team was an Aatrox, and their team had a Fiora. Apart from her being way too strong (some would say broken) right now, she has a bug. long story short Aatrox tried to 1v4 under tower, and Fiora ulted him. She had only hit 2 vital points when he went into passive, but her heal still went off. Aatrox was not technically dead, and she did not hit all 4 vitals, so it makes no sense that her heal should go off. And yes, I slowed down the video and it shows she only hit 2 vital points.