The database call says it comes from the person:
DbImmediate( 'select &AO.PERSON_INFO_API.Get_Name(:i_hWndFrame.frmIdentityInvoiceInfoSupplier_Ext.dfInvRecipient), &AO.SUPPLIER_INFO_API.Get_Name(:i_hWndFrame.frmIdentityInvoiceInfoSupplier_Ext.dfInvoicingSupplier) into :i_hWndFrame.frmIdentityInvoiceInfoSupplier_Ext.dfInvRecipientName, :i_hWndFrame.frmIdentityInvoiceInfoSupplier_Ext.dfInvoicingSupplierDesc from dual' )
However, that is version 9, yours must be version 10. Run the debug console and look for the line that calls that information. The field just below invoice recipient says where to pull the recipient from, does that offer a clue?
V9 defaults to the Person and the entered supplier which is logical as you are assuming.