What could cause me to miss getting the achievement for beating the AI?

I just finished a 4-hour game where I beat a 10p FFA game against all nightmare AI's.
But I didn't get the achievement for it on that map.
What could cause that?

I did press "Continue playing" when I got the victory.
Might that cause me to not get the achievement?

I just did it because I want a saved game of the endstate so I can check things later on.

Why would that cause me to not get the attribution?

Is there something else I may have done?

I do save/load a lot, but in other games where I did that I did get the achievement.