If Stax Bill cannot create the invoice in QuickBooks Online the first time, it will reattempt to create the invoice and not include the invoice number in any follow-up attempts. This allows QuickBooks Online to generate its own invoice number.
To correct this, the number can be changed in QuickBooks Online: https://quickbooks.intuit.com/learn-support/en-uk/manage-customers-and-income/how-to-change-an-invoice-number-video/01/239358#M20