Because of the delay between Holy going off and the Stun taking effect. The pull before the second boss of the Twinning is a good example of this. AoEs will go off before your third stun and I'm not really sure what's given you the idea that they don't.
I'll make a recording tonight as I think it's the only way you're going to see straight on this <3
If you're wondering why I'm making a point of this:
It's because this is patently untrue.