I got all 18 aesthete stars, but the achievement did not pop. Looking in the achievement menu it says I only got 13. Given that Sejong popped when it should have, at 12 stars, I think those reported 13 are the first 13, and the rest weren't counted. Victory video showing that I got all 18 stars: https://drive.google.com/file/d/1q5rTvWaHaAT8hFUMViQpno6vtZZrfxn7/view?usp=sharing

Having played around with this a bit more, it looks related to loading saves. If I go for one of the 18* achievements in one sitting, they're awarded, but if I get a few stars, save, then later load the save and earn the rest of the stars, it won't. Watching the progression of the steam achievement bars suggests that if a save is loaded, the progress towards the achievements made up to the save point is not counted, and the counter starts from zero again 

I'm having the same problem with the "Napoleon" achievement. I got all 18 Militarist stars but steam is only saying I got 16 out of 18.