That's just how latency works. If his animation has started and you flash, you were in melee range and it will hit... even though you see you flashed, the servers don't, I can almost guarantee it. Generally speaking, I don't even think shielding with Kayle will stop an initiated ult.

Worst case scenario, the game just assumes you were in melee range when the ult initiated and cannot distinguish that you are no longer in range, as you had to be in-range for the ult to trigger in the first place.

My suggestion is to flash before he ults.

still fake as hell if u flash away nobody can reach u hitting u with an AXE!.... this is the kind of things why Darius is broken and even his teamates hate him. he is a scumbag champ that can own u even when u play much better than him.

You can't avoid incoming damage with flash. If you flash when Maokai is using his Twisted Advance he will still follow you and deal damage. If you flash when TF throws a yellow card, you still get stunned and take damage. Once the ability is in motion that's it.

if viegar ults you, and youre dumb enough to flash after the animation starts, why shouldnt it hit you?

same applies to darius, shouldve flashed before he stacked you up

Yeah it's just counter-intuitive because the ability makes it seems like you have time to dodge after it starts, but the animation just lies. If he's in the air, the dunk is coming. Moving the hoop makes no difference for Darius. So it makes Darius even more frustrating when it happens because you think you cleverly used your reflexes to dodge certain death, but he just dunks from downtown.

I don't think that it's a broken mechanic or a bad one, just a frustrating and counter-intuitive one.

The ability makes sense when you compare it to a Brand ult or a Veigar ult, but it doesn't make intuitive sense, which is why it feels bad for it to happen to you. You know that Brand's fire or Veigar's magic will home in on you no matter the distance, but you never expect Darius' phantom axe to decapitate you from range.

TL;DR Darius' ult interaction with Flash is logical from in-game mechanics standpoint but counter-intuitive to the average player. This makes it annoying but not broken.

very shortly after darius casts his ulti it will be unable to be disjointed (possibly even by finishing to recall if you're timing is that unfortunate) by distance or anything short of a stun / silence on him.