Vi's Ultimate Being Stopped/Interrupted

For the 2nd time this week, Vayne's condemn has interrupted my ult and knocked me back even while I'd been targeting her for at least 1 second. In the ult description it says "While charging, Vi cannot be stopped" and the LoL wiki says "Vi becomes immune to CC" which I believe is supposed to be correct. But Vayne's Condemn has interrupted/cancelled/stopped my ult twice this week.

I play with 49ms and rarely lag so I am 99% sure this is a bug and not a lag or connection issue.