Did you try reproducing this with latest IS core fix levels installed?
Capture the request and response xml by enabling to TRACE, which will help you to analyze the issue? You can also use TCPMon to intercept the request or response.
To check if wM issue or not, can you load the consumer WSDL to Soap UI and see you get the expected response?
#webMethods-General#webMethods#Integration-Server-and-ESB