Probably a newbie-sounding question, but whenever my NPCs HP hit 0, rather than dying, they just kind of... Stand there.
I haven't tested .Died with it yet, but is there a way to get them to die in a similar way to the Player's death? (i.e falling apart)
I believe that the problem is that your NPC is anchored, if you import an NPC using a plugin or free model this happends pretty often, unanchor every part (HumanoidRootPart too)