I'm running only a modestly modded game right now and have UFO running for the follower options, but I'm encountering something strange.

Vanilla NPC's (Jenassa, Erik the Slayer, Uthgerd, etc) sporadically gain an essential flag out of the blue. They can start off normal, then one day they're unkillable, and I can't get the flag to drop, no matter what I try. Anybody else encounter this?

Essential followers are my personal bane.