Rengar bug?

I was just playing a game vs a Rengar and every time he would ult, I would flash it, but the damage would still be applied. I understand that if his auto attack animation has already been registered it would still apply the damage to me. If you want the video though, you will see that Rengar is at the beginning of his jump, and I flash and still the damage is applied to me when he lands. This doesn't seem right to me. It basically makes it impossible to counter Rengar at that point. His ulty doesn't reveal the exclamation point until he is basically in jump range and if you can't even flash out of his ulty, then how do you counter this?