Nautilus Passive verse Dash abilities

So i was playing a game as Nautilus and i am not sure if this is a bug or intended but when I auto attack a Champion namely Lucian because i was bottom lane. Shouldn't Lucian not be able to immediately dash or at least shouldn't my root apply after the he stops moving instead of during his dash. This may be intended but i would at least like some clarity.

To Recreate: Just play against Lucian and try to root him and have him dash right as you do. The passive will apply (you can tell by cooldown ring showing) From my screen it also looked as though your passive can be on the Lucian and he can still dash.

PS I haven't tried this with other champions. Also if this isn't a bug then i am sorry for wasting time