So, what’s wrong with that? They’re not unstoppable, so you can stun them.
At 0.02, Diablo Knockbacked your hero with Shadow Charge before the cast finished, as shown by the yellow Knockback animation above Ana (Ana successfully started the 0,25s cast, but it did not finish).
Sleep Dart is not an instant cast. It can be interrupted.
If it hadn’t finished casting, you wouldn’t see the sleep dart flying past him and I wouldn’t have a cooldown.
I should’ve been hit by his Q, but he also should’ve been hit by my E.
I think this is the same bug as Genji E (and maybe Tracer Q? haven’t tested that one), where their hitbox and the sleep dart hitbox don’t touch for long enough for a gametick to recognize it, so they pass ‘through’ it.