I'm not sure I understand still.
It makes sense that NPCs are not spawning directly next to your character, since they are programmed to spawn off-screen.
I do not, however, understand the example about the Slime. So, you go down a cave. A slime spawns. Allright, so NPCs do spawn. But instead of the slime behaving naturally, you could say it falls through the ground, never colliding? This is what I can make out of your story.