Sion ult bug

Hi guys

Today in a game i occurred a Sion ult bug, or so i believe. I was in lane vs a Darius. I was stood directly on top of him (to the point of being champion blocked) and i pressed R. My ult proceeded to bounce of him instead of knocking him up.

Its hard for me to show where my mouse was at the point of pressing R. So I have tried to make it as clear as possible in the link as follows. You can clearly see at the point of me pressing ult i am facing Darius but then a few frames later you see that my direction has changed completely.

Link: https://imgur.com/a/JCp7Hzw

I would love to know what you guys think about this.

Sorry as this is my first time posting something like this on reddit