WHMCS Provisioning gives error on recurring billing

I'm using the WHMCS Provisioning plugin and it's working well. When a customer places an order, their site is set up.

However, each month when the customer is successfully billed, I receive an e-mail from WHMCS saying "WHMCS Automatic Setup Failed" and in the body of the e-mail, the reason is "Mapped domain already exists".

Obviously the domain already exists because it was already set up for that user. What doesn't make sense is why it's trying to set them up again.

Do you know why this would be? Is it a bug or what?