Summary: Kapaku faction does not pop the "United Empires" shared victory achievement on steam.

Details:

I have completed both a shared Quest Victory as well as a shared Expansion Victory with the Kapaku faction on a steam account that had not previously unlocked the "United Empires" (shared victory), "Loremeister" (Quest Victory), or "Bigger Is Better" (Expansion Victory)  achievements.  On the victory screens, shared victories were explicitly indicated in both games.  Both the Quest and Expansion achievements popped, but the "United Empires" shared victory achievement never popped.

[Note, the above games were both single-player against AI, not multi-player.  Also, no mods. I was playing as the Kapaku in the shared victory, not the AI.]