This could be a case of partial payment. The Amount Due, i.e. the amount left to pay (calculated as the Bill's amount due minus the Bill’s payment amount On Approval minus the Bill’s payment amount In Processing minus the Bill’s payment amount Awaiting Payment), gets updated during the next synchronisation with Xero. If the due amount is not equal to zero, it means your Bill has not been fully paid yet and therefore still has the Awaiting Payment status in Xero:
This is how the amounts are calculated:
Amount Due:
obtained from the Xero Bill data
Amount On Approval:
calculated as the Bill’s payment amount which is On Approval within an Airwallex Batch Payment or another Batch Payment
Amount In Processing:
calculated as the amount of all Bill payments which are In Processing
Amount Awaiting Payment:
calculated as the amount of all Bill payments which are in Airwallex Batch Payments with the status Awaiting Payment