issue with xin's ult

Recently, I've noticed that Xin's ult does not affect every champ. Although there have been a few who've I've seen this with, my recent encounter was against a riven. To my understanding, the ult should cause a knock back effect. However, Riven (and several other champs) are not being knocked away or even stunned for that matter and can continue their barrage of attacks during Xin's only line of offense defense. Im not sure if this was meant to play out this way, or if there has been a bug in the game for a while now.