Stability/Stun Breaker.
it’s a bit annoying yes but it’s been this way forever, on the one hand you could say this is annoying and should be fixed, on the other it could come down to you mistimed the skill and suffer the consequences.
I think of it as a bug, cuz if you haven’t been knocked down yet, it should prevent you from being knocked down. Either you’re knocked down or not. The half-way thru being knocked down — that doesn’t make any sense.
Having to go thru the animation to fall down before you can get the benefit of your shout is an awkward bit of ability lag (which there’s a lot in this game).