Invincible rares bug

Several times I found a rare mob that was absolutely invincible, one of them was untargetable so I couldn't see its affixes. In both cases I made sure it's not an affix like proximal tangibility to prevent the monster from taking damage from a far. There shouldn't be anything else that would cause this. It might be worth looking into. I had to give up my map, as it was unkillable.