3176257 - The ERPPOLineNumber tag in my receipt export XML is empty

The <ERPPOLineNumber> tag in the receipt export XML from my SAP Ariba Buying site is empty or contains no value. I notice the same issue also affects the invoice export, where <SAPPOLineNumber> is not populated in the invoice XML. 

ERPPOLineNumber, SAPPOLineNumber, receipt export, invoice export, purchase order response, ERP integration, SAP Process Integration, empty tag, force order, cXML, middleware mapping , KBA , BNS-ARI-PUR-RCV , Receiving , BNS-ARI-PUR , Purchasing , BNS-ARI-PUR-RCV-IEX , Receipt Import/Export , Problem