[GAMEPLAY] Couldn't move off of Rek'Sai tunnel

I was playing as Alistar in a normal game, and while standing on a Rek'Sai tunnel to destroy it she ulted to that tunnel. While the animation for her ult was going on, I was unable to move off of the tunnel which led to my death as it gave their team enough time to surround me.

Note: I do not own Rek'Sai or I would test this out further to see if this is a one-time or a consistent thing.