I see you added an option to set the coins to 0!
I set 300 coins and then exit the app to home menu. They are showed. Great!
I went inside the app again and set it to 0. Exit to home menu and it showed 0. Perfect!
The only issue I´m having is when you set it to 0 or 300 and without going back to the home menu you set it to 0 or 300 (depending on your 1st selection), the app crashes, then takes you back to home menu and restarts the system. (maybe it's normal because it needs to refresh the counter).
PD: I'm going to format my SD card then try again. Because sometimes I am having this weird issue were the 3DS would hang ''closing the application'' exiting from the save manager.