Possible Vi Ultimate Bug?

I know Vi is supposed to be pretty much unstoppable during her ult, but this interaction struck me as odd all the same and I'm not certain whether or not it's intended...

A friend and I were testing out junglers against each other in Twisted Treeline in a custom game, he on Elise and I on Vi. At one point, I went to ult him, but he hit me with a cocoon and I was stunned but my ultimate went through all the same. And I feel like if he hit me after I started the channel, then it would be normal that the ult would go through, but I also feel like the stun from his cocoon wouldn't "activate" in that case. I know, because I have the settings in such a way as to notify me, that I WAS STUNNED but my ult went through regardless. I'm not sure if it's a timing issue or not, but it seems that if Vi is confirmed to have the status effect of "stunned" that it means that his stun went off before my ult and that my ult wouldn't have gone through (and wouldn't have gone on cool down). But this was not the case. I ulted him while stunned. If nothing else, I feel like this is a bug in the text? Like, if I indeed was stunned after the initiation of my ult then I should have an "immune" effect appear instead of a "stunned" effect. But then again, I was stunned for a bit after my ult completed, so I feel as if I was actually stunned instead of it just being a textual error. Sadly, I don't have a playback of the game so I can't be certain which was cast first - his stun or my ult. But it just strikes me as odd if Vi can be hard-cc'd during her ultimate. As far as I know Malphite and other owners of "unstoppable" ultimates have CC immunity during their ult, and I thought it was the same for Vi. So, the possible bug is either: Vi can be CC'd during her ult, the text that should read "immune" reads off the intended CC instead, or despite Vi being CC'd right before (or possibly the exact moment she ults) her ult goes through as if the cast was uninterrupted. If this indeed is working as intended, it strikes me as odd... If nothing else, if Vi is indeed unstoppable during her ult, then she should be granted immunity to CC during her ult. If this is already the case, then this instance is indeed a bug...