[Bug][Gameplay] Sion "Unstoppable" ult interrupted by Diana E

To my understanding Sion ult describes "While charging, Sion is immune to all Crowd Control."

So I was playing a game and I was trying to backdoor enemy nexus by rushing past. I was like coo'coo' I got this guys. Got close enough that the ult could make it the whole way to the nexus when a wild Diana appeared in front of me obviously trying to block my ult.

I simply walked around her and started my ult and she ran after me. I got about 6 steps into my charge and my screen had already switched to the ult's cam and the drive mechanic took over with all the whooshy sonic boom effects when she fired off her E. My ult was immediately stopped and I was pulled towards her and slowed leaving me baffled as to how she double CC'd me while ulting when Sion's ult is an "Unstoppable" effect like Vi, Malph and Hec's ults. I certainly didn't collide with her, because she was behind me.

While I still didn't care I just E'd her and walked the rest of the way unchallenged and won the game. I figured it was note worthy to still report such a thing happening so it can be fixed. Either being his R or her E being the problem.