Adding Attachments to Purchase Orders Created via Requisitions Using Web Service

We are trying to add text as an attachment to open status purchase order headers and lines created through requisitions using a web service. We used the "uploadAttachment" operation for "ErpObjectAttachmentService" to achieve this functionality. However, as per the Oracle notes, this operation works only for purchasing documents created by import or web service.

When we use this web service to add an attachment to a purchase order created via requisition, a blank change order is being created. We need assistance in resolving this issue so that attachments can be added without generating a blank change order.