Unkillable Monsters in Cruel Mastodon Badlands

Cruel Act 2, Mastodon Badlands.

A Rare monster appeared that spawned a number of spider minions. Killed the Rare monster, but the spider minions persisted indefinitely. Spider minions were untargetable and would not take damage from any skill or attack, including AoE, explosions, and DoTs. They followed me, continuing to attack and nearly killed me a number of times before I left the area.