I’m not sure since I haven’t coded a thing in my life, but reading the symptoms tells me that they’ve coded in some ‘maximum’ for NPC’s in any given area. Perhaps in an attempt to prevent technical issues, or simply to prevent NPCs from eventually outnumbering the zed’s given enough time.
I’m sure though that since you can go to a new area and find new NPCs there that you can ‘import’ NPCs into your original area purely by having them follow you to whatever base there is.
Seeing how buggy NPCs can be, I have no issues with a maximum of spawneable NPCs in a certain given area, knowing they’d probably end up torching everything they see with flamethrowers.