Bltzcrank Hook Bug

So recently I have noticed that when I play support against certain ADCs, especially and , they can use their mobility spells in the middle of my hook hitting them. So my hook hits, they start to come towards me, but then they can just escape with their Rocket Jump and Arcane Shift to escape my hook even though the description on the LoL game information page says that "Blitzcrank fires his right hand, if it encounters an enemy unit it will STUN them and deal 80/135/190/245/300 (+100% Ability Power) magic damage while he pulls them to himself. Last time I checked neither of those are cleansing abilities and mobility spells have no special clause regarding stuns. If someone could help clarify this issue for me that would be great. Thanks!