No root on deadly flourish?

[GAMEPLAY]

I don't know if i'm seeing things wrong, but it seems as if deadly flourish doesn't always root my target even when if the target is marked. It's not a one time occurance, it happens randomly and the damage of the ability is still dealt, but the chamion is not rooted. I'm currently playing with the high-noon jhin skin, so it might be a problem with the marking of enemies on that skin?