I played as Khatep on Vortex and built the "Sanctum of Quintex" T5 building in the Ancient City of Quintex settlement and the achievement did not pop. I destroyed the building and reconstructed it all the way from "Vault of Quintex" and still nothing.

I was able to get the achievement by playing a non Tomb-King race. I noticed that the Sanctum of Quintex building has two different variations, even though the share the same name. One for Tomb Kings, one for everyone else. The Tomb-King version of the Sanctum of Quintex does not seem to prompt the achievement, so my assumption is in the code it has the same name, but is considered a different building for the sake of the achievement.