They have become a little more "invisible " than usual so probably buffed them with last hot fix .
I have noticed invisibility not working that great hardly revealing any I have volos eye and zombies seem to be struggling with detect presence.
I do Aoe the 4-6 areas with fire,Ice,tentacles,thorns in hopes I'll catch one.
Yurgir was ridiculous the other night 4 of us surrounding him and not detecting him.