Possible Sion ult bug (Fizz/Yasuo ult interaction)

So there I was, playing the Undead Juggernaut in the top lane. This particular match had been going on for quite some time, and I was in the process of splitpushing in order to demolish one of the enemy's inner towers. I was giving the the structure a good thrashing, when suddenly a wild appeared, followed by a . Being the dark unstoppable war god that I was, I shrugged off their piddling damage and smashed the tower to dust, and proceeded to gloriously ult away to safety. However, as I began my heroic disengage, the Fizz managed to land his ult right underneath me; of course, I thought nothing of this, for I would be immune to all crowd control for the duration of my majestic flight.

As experience had taught me, the Fizz ult activated with no effect. I allowed myself to gloat for just one moment; truly, it was a thoroughly triumphant play. But my brief celebration was cut short! Imagine my shock when the Yasuo ulted straight to me, halting me in my tracks long enough for Fizz to catch up and finish me off.* Outrageous fortune! How could this be?! The only possible conclusion in my mind is that, despite being immune to the knockup effect of Fizz's fish, I was still somehow considered airborne, allowing Yasuo to ult and interrupt my charge.

*Naturally I got back up and slew them both for the double kill.

tl;dr in the middle of a Sion ult I was ulted by a Fizz, and despite being immune to the knockup, Yasuo was still able to ult to me and stop my Unstoppable Onslaught, wtf rito fix ur spaghetti code gaem