Illaoi E Being Cancelled

Playing in slow motion, you can see me enter the cast time for the E, thresh's hook lands, the animation stops and the ability goes on cooldown. Albeit the cooldown in the corner is pretty buggy in slow-mo, you can clearly see it start a second after everything goes down, which happened immediately in game. As far as I was aware of, using Illaoi's E is not a channel and should not be canceled by hard cc. This is not the first time this has happened to me, either, just the most recent time.