Server's next payment date isn't updated when invoice is fully paid from balance

[Kirill]

We've experienced an unexpected behavior this billing cycle. Some of our customers refill the balance in advance. For those invoices that were fully paid from balance, next due date wasn't updated. If the invoice was partially paid from balance or fully paid with any payment method, next due date was updated correctly. Any idea what can be the reason? We're using WHMCS 7.5.1.