I was in ACTIV and I killed a rare (yellow name) monster with invulnerable minions, I'm a Demon Hunter so I keep my distance, when I killed her the 2 minions around her also died, HOWEVER when I continued my progress to where I first encountered her, the few minions that did NOT follow her, were still there alive and kickin with invulnerability, arcane enchanted, and devastator... not cool.

Side note* I've also noticed I've been able to kill invulnerable minions occasionally before killing the unique, haven't noticed what it is that is doing it, but will keep a look out.