I've never seen or heard any reports of players experiencing this coin issue in BS so I think you should be okay.
Occasionally in DS, when your coins are in the billions, the game can sometimes appear as though it is subtracting coins instead of adding them. But even then the coins aren't actually being taken away from your account, it's just that the counter shows the wrong number temporarily. When this happens, the issue is normally fixed with a simple force close of the app without any lasting effects.
On the off-chance that you do experience any issues with your coins when the time comes, you can
open a Support Ticket with the team who should be able to help.
Good luck, and congrats on the impending achievement!