The Elf Overseer will only spawn if the World Change is reset by extinguishing the fires of Shadowthorn.
I tried to kill 5 Elf Overseers in two days. The first day I went there on the reset of the Thornfire World Change right after server save, killed the 3 Elf Overseers and waited for the next server save to see if the World Change was triggered or not. Nobody removed the moss on the prisons, so we had another day of normal Shadowthorn. On the second day right after the server save they weren't there. They were also not killed by another player because otherwise their corpse would be on the ground.
So to be able to find them again, you must burn Shadowthorn and then extinguish the fires so that in the next server save they will respawn again.