Boss in Waycrest Manor Bugged Mythic Keystone

Just did a Waycrest Manor +2 - first boss we went was the witches. One witch became invincible. We could still damage her, but she wasn’t dying. We kept attacking her for ~2 mins and decided to just reset the boss. She had what looked like a dark grey hp bar.

I was able to repro the issue a second time.

I think it’s related to Monks and Touch of Death. Somewhow, if you lower two witches to very low health and when the one that has the Iris is about to die, you can touch of death it to kill it. Then when the second witch is “taking the Cursed Iris”, you can touch of death her and she will die before she finishes her cast. So that leaves the last WItch which technically doesn’t have the Iris so she can’t be killed because the second witch never finished casting it.