Walty

First i thought it was a bug game, but then i tried to switch server (europe to US server) and i found Seff in the dungeon.

 

So, i think that exist a long Seff's respawn  (too long) once someone complete this quest. Probably you'll find Seff there in few mins or hours later...

i think thats the case with the npc spawn. i came back after 12 hours, he still weren't there but i found him after switching to US server

so the solution for anyone that's gonna have problem with this quest: switch server