[GAMEPLAY] Bug: Sion R interrupted by CC

Sion's R, Unstoppable Onslaught, is intended to make Sion "immune to all crowd control". However, I have had at least a few instances in the last month where it was interrupted by hard CC. Not terrain, and not by reactivation of the ability.

The first time in recent memory where this happened was versus a Lulu. I pressed R and began to charge. After a second or two, she used polymorph on me. This is supposed to have no effect, however it successfully turned me into a squirrel. At first, I kept charging. However after a short delay, the game seemed to decide that because I was a squirrel, I wasn't supposed to be charging, so my charge stopped early.

The second time was versus a Janna. I forget how far into the charge it was, but I think it was not very far, i either just pressed it or it was after 1-2 seconds, then Janna was behind me to my right. She ulted, knocking sion a little forward, but also a little sideways. This caused my charge to end prematurely.

I know it happened a third time, but I'm not sure what caused it, as my screen was either just coming back to Sion, or I wasn't looking directly at myself. When I came to realize my ultimate was interrupted, there was no champion directly in front of me that I could have run into, and I was not directly adjacent to terrain. However there were champions nearby, and they began to kill me.

So, it doesn't happen very often, but it appears that there is something wrong with how the ability is programmed, and I don't want this to happen to me again during a clutch moment where it shouldn't.

A common answer will probably be "you hit terrain, noob" but with over 100 games on Sion, I can say I have a very good feel for it. A possible reason that I propose, is that Sion's charge is considered to be a channeled ability. And after pressing R, there is a windup period before he really begins to charge quickly. I think that sometimes, the crowd control immunity fails to "catchup" and is not applied to Sion instantly, being delayed, thus allowing for a window for him to be crowd controlled. Either this doesn't happen all the time, or is only a weakness from the back/sides, as I remember being right in front of a cassiopeia, and snap-casting my ult just as I heard her scream, saving me from her petrifying gaze's CC and allowing me to charge a few steps into her. I would look into Sion being targeted/crowd controlled from the backs/sides during his ult, and/or early on in the ult. As two out of the three times where Sion R was interrupted prematurely happened from the back/side. Actually, I think they were both when I was charging West/Northwest, and interrupted from the Northeast. Both in mid-lane, actually. (based on camera angle, not lane placement)