[GAMEPLAY] Knock-up while in Zhonyas

During a Game I played as Blitzcrank I hooked a Rumble that aktivated his Zhonyas almost instantly and I pressed my e and got my AA through I knocked him up, but dealt no Damage to him even he was untargetable. I do not know, if this occures only with Blitzcrank or with other Champs aswell. That is not gamechanging but still isn´t supposed to happen.