So I’ve unlocked both the hard level for Chicago and Chongqing but the Google Play Games achievement didn’t unlock.
This happened to me too, I’ve gotten more than 500 passengers several times but the game hasn’t given me the achievement.
This happens to me too… however if I go to PLAY-> Chongqing and check the badge there, I can see my Chongqing achievements. Eventually, this will sync up with Google Play, and then they will appear with all your other badges.
I finally got synced up again yesterday and this cleared up - I also could access the leader board again, so it was a good day for Mini Metro.