Greyed enemy health bars

Hey all, I dont know if this is a new thing or if Im being a bit of a dummy, but I've noticed since the update that some of the enemies' health bar will periodically 'grey out' and the enemy will not take any damage whilst the bar is grey. 

It seems like it only happens when the enemy is being stunned or has been hit with a combo like electrocute, and when the enemy is stationary. 

Can anyone shed any light on this?..  I havent heard anything about it and cannot find any info on it in the forums.

Cheers!

 

Electrocute shouldn't effect "grey" health bar or taking damage. I think you accidentally proc the new combo Petrification, which can be accomplished by applying EARTH damage onto POISONED enemies. 

So if I understand what you're saying, its the Petrification combo that causes the health bar to grey out? So the enemy cant take weapon damage while being petrified? 

They take damage but the bar doesn't move until they unfreeze and while grey other mobs will attack them. So it is common to see a boss go from full to Dead :) 

Aah I see..  Thanks for clearing that up. 

see it here Greatly increases CD for highest DPS player-controlled unit, and reduces the RoF, armor and evasion of the lowest and highest HP player-controlled