@Alexjb17 @Wolfie557 @NecroreaperGR
I managed to get it!! I did this:
Loaded my last savegame. At that point, I’d already defeated gray and yellow. I declared right away Persian Outpost as enemy, but did not attack them. I keeped attacking red, then blue, and happened the same as before. Aquamarine resigned and seconds later blue did the same. Instantly the achievement popped up! It seems that the scenario did not automatically ended because Persian Outpost was still my enemy. I changed it back to ally, and the scenario ended.
It seems the problem is that the scenario ends too fast when last enemy is defeated, so it has no time to check the condition and trigger the achievement. Having Persian Outpost as enemy allows the scenario to keep going so it can trigger it. Once you have the achievement, you can change it back to ally and then the scenario ends and victory appears!
Obviously this is not the way it is supposed to work, but at least there’s a workaround to still get the achievement.