Inconsistency with Duskblade

So, using Duckblade Duskblade, I've noticed something weird about the damage-burst passive. Allegedly, you have to be "Unseen for at least 1 second." But Vanish type abilities, especially Rek'Sai's ult, don't seem to activate the Passive even though she vanishes for 1.5 seconds. This doesn't seem right.