why rengar's jump can't be stopped by Cassiopeia's w ?

its so frustrating to lane against. your w promises you that it will stop all dashes and jumps yet when you lane against rengar it doesn't stop it what so ever. he just constantly jumps out of the bush or while stealthed even though he's grounded and you're forced to duel him and by then you lost an important part of your poison combo and that w is like a fuck you in your face seriously why hasn't riot noticed this yet?