Katarina became untargetable during "ult" after Reksai's Knockup

I recently encountered a "bug" while playing as Katarina. Explanation kind of confusing. Sorry.

Here's what happened: There involved 3 players : Me(Katarina), Enemy1(Reksai), Enemy2('Random_Champ').

First, Reksai used his knockup under me right as I Shunpo + Ult to 'Random_Champ'. So in theory the knockup should have either missed or cancelled my ult (it has to be one or another), but It didn't. Instead it popped me in to the sky (kind of like the Blitzcrank grab person across the map bug, years back, the person got pulled to the 'ceiling').

So I didn't come down for about 67 seconds. During that my ult continued with no hinderance(my character can not be targeted). Basically I became a Katarina with cc-immunity during ult(nerf please?) for 67 seconds.