The Catalog CXF Method getRequestAttachments does not return the URL tag. This does not occur when using AXIS framework.
getRequestAttachments method is part ot the Request Service (http(s)://CATALOG_HOSTNAME:PORT/usm/webservices/RequestService?wsdl)
Service Catalog 17.4 RU2 to RU5
This is a defect with the product.
Submit a case with support to request a fix that is available for 17.4 RU5. The fix will be included in future releases (as of now, future releases are 17.5 and 17.4 RU6)