Some large enemies become temporarily invulnerable

I've run into several large enemies in Act 4 that sometimes become invulnerable when frozen and cannot be hit by the player or minions: Subdued Behemoths, Elephant Tortoises (sorry if I got the name wrong), and the big beasties that carry a bunch of kinfolk on their backs (sorry I don't know the name of that one at all). I've tried circling around them to hit them from different angles and using AoE attacks, but they are just completely invulnerable until they thaw out. It doesn't happen every time, though. Only maybe 1 in 5.

UPDATE: After further testing, it seems to have nothing to do with them being frozen. It actually seems to be caused by their position. It just makes it more noticeable when they get frozen in a position where they can't be damaged because they remain invulnerable for several seconds, whereas when they aren't frozen, they tend to move out of that position quickly. It seems to happen when they are in narrow passages and choke points.