Wrath isn't instant. It is fairly fast, but it isn't instant. Actually, the animation (roaring to the sky) seems to last longer than the cast-o-meter. Another bug? I'm pretty sure Ooze is instant, so you may be confusing the two.
Actually, this could be pretty killer. If you can use Wrath to cancel Grasp, then you cancel Wrath, you would be able to punch him in the face while he is stunned. Does this work with other casts? Venom spit is a lot faster and thus could prove more useful. Ooze is instant iirc, so if it just stops the Grasp you would be free to punch him in the face right away. And you could already have Wrath activated when you do that. Hmm, I'll have to experiment.