Hello
I just got to 500 residents in Arid Highlands as a Tier 3 on Vanquisher, and i didn’t get the achievement?
In the same game i didn’t build any crop farms. I did build Aborists, but they dont count as crop farms? I have upgraded to tier 4 afterwards trying to get this achievement.
Welcome to the forum. 
Did you change any settings under Advanced Settings or use Custom Settings at all?
No, arborists don’t count as crop fields; you can use those just fine.
Hello Medea
No i didn’t change any settings.
I got the Highlanders achievement in the same game 
Stupid me.
I started it on Trailblazer.
You can just close the thread 
It happens, no worries. Of course now you have to try again on the correct difficulty. 