Cassiopea Ultimate Bug

I discovered a bug while I was playing Cass today. I was playing against a Kalista and I tried to ult her. At the beginning of my ultimate animation, she jumped behind me and didn't get stunned. The weird part was that when I moved my mouse to track her for a follow-up Q, I ulted the opposite direction, towards where my mouse had moved. I just thought it was strange that it could move. I tried it again and it was repeatable. I'm not sure if this has happened to other people or if it was just my computer lagging a bit, but just a heads-up for those who play Cass.