Hostile NPCs Disappearing?

I’m playing with Kenan’s Modpack, and while fighting the now common hostile NPCs I lose line of sight and they disappear into thin air. Poof. They don’t even appear on the map. Is this supposed to be the Ambush Mechanic, or is it possible that this is a mod being funky? Here’s a pic for proof:

You can see that I shot him, he ran up to the north of the building, then disappeared as soon as he rounded the corner. The helmet was what he was trying to wield after losing both arms. What gives? I want his fat loot.

Edit: moved to Garage