Monster Infinitely Respawning After It's Killed

On few maps recently, experiencing one or two monsters infinitely respawning after it has been killed...over-and-over. Sucks when they are melee - they keep chasing you. You either lose the map or try not to die while facing an immortal monster, lol.

most likely they were from a rare pack, some rare monsters can resurect their "minion" you must kill the rare to be able to kill those monters