I'm pretty sure that when you take a big amount of damage at the same time as you use the 3, Divine shroud, the cooldown is set to 0 and the shield was used. I have not noticed this when Argus was out at the same time however. for Example a the last time i played Hera I got hit with a Janus ult at the same time as pressing the shield( the animation did go off). I received 700 damage or something and had only lost 300 health, but the shield was ready to use again as I did use it in the next 4 second , where it should have been on cooldown. I was also not attacking anyone so I'm pretty sure it wasn't her passive. Don't know whether this is intentional or not but it isn't stated in the ability description. I think this has happened to me before.