On the 1.1 opt-in beta, with a custom faction created in-game on 1.1 at the beginning of the game, without saving and reloading at any point (I used a tiny map to try to recreate), I got the achievement. So now there's no way for me to do any further testing.
If I recall correctly, all my previous games that had failed to award the achievement had been multi-day games where I'd saved, quit the game, and then loaded the save another time, and of course all had been on code levels lower than the current 1.1. beta.