Cassiopeia Grounded vs Zac

Cassiopeia's grounded cancels movement abilities and prevents people with channeled ones from casting it like lissandra. However, I have noticed at least on Zac's bounce that he is completely unaffected by it. All it says when you cast it on him is cannot be slowed but that is not the point. If he is completely immune to cc I suppose that would be different but I think its a bug