This is not a bug. Every time you are suppose to gain diamonds you can see them at rewards of your quests, deals or any kind of activity you are doing in the resort.

But sometimes you can see a diamond flying toward a Piggy Bank located at right side of your screen. This Piggy Bank is not actually diamonds that you earned, but some activities around the resort like completing quests, deals or sending Cargo Ships will fill this Piggy Bank so you can further purchase it for a fair amont and receive a quantity of diamonds depending on how much you filled it.