Unhittable mobs during breach

Not entirely sure if this is a bug or really bad luck. I did a breach on a very chill t15 map. Once I opened the breach nothing would take any damage, I ran around in circles continuously attacking and nothing took any damage. This continued until there were so many monsters I couldn't move and died. As far as I have noticed the rares temporary invulnerability only has an effect on its minions.
Nothing would take any damage, whites, blues, rares. I think this is a bug but things usually die so fast I cannot trust whether I actually know the mechanics of temporary invul.