Sultans Ascend Campaign Steam Achievement not working

I finished the campaign, I even redid the last battle several times. Still, I don’t get the steam achievement for it.

I saw that other people have the achievement. So apparently I am doing something wrong. I finished the last map and I didn’t get it. What do I have to do to get the achievement?

Somewhere I found the info that you need to play it in one go (i.e. you can’t stop and then continue on the next day). Tried it, still didn’t work.