What happens when NPCS "go away"?

Game mode: [Offline | Singleplayer]
Problem: [ Misc]
Region: [Italy]

I started build things near the Yakira’s temple (Derketu’s, near Xe La city) first I managed to talked to her
then I started build stuff “in the back of the peak”. Also another named NPC (an Archer) was there and I pay attention not to kill him, anyway after I buils a certain amount (?) of stairs and floors she (Yakira) and all the other (hostile) cultists that were with her (included the named archer) simply vanished.
I already noticed that in previous trials, but are they lost forever?
I mean who cares of the general npcs, but the named one ?
Are they simply erased from the location?Will they spawn somewhere else?

Thanks in advance for the replies

Steps on how to reproduce issue: