Vaal Molten Shell Crit Mechanics

Iam currently playing a character thats using Vaal Molten Shell as its main damage source. I experienced, that the chance to crit with the explosions of Vaal Molten Shell is either 100% or 0% through the whole duration of Vaal Molten Shell. So it rolls obviously for the first explosion if its either a crit or not, and the following explosions won't roll for themselfes again. Is this supposed to work like this?

intended, the crit rolls are done on cast, when you activate it...and will not roll again until you activate the attack again.

This why they nerfed the fuck out of crit a while back and removed the old diamond flask