WebService call from an orchestration anomaly

We have stumbled into this strange behaviour a few weeks ago, but now Microsoft I believe confirmed this to be an unexpected behaviour so I've decided it's a good time to post it -

We have a web service we're consuming from within an orchestration in BizTalk.
This web service has two methods - one is two way and the other one way (it does not have any return value)

We use the SoapDocumentMethod attribute on the web method to make sure it is a true one way web method ([SoapDocumentMethod(OneWay=true)])

When we add a web reference to this web service from the orchestration we get a web port generated with two operations as expected, one is a request-response and one is a One-Way (send only).

However, strangely enough, when we send a message out through the one way port we get a routing failure message in the message box.

What happens is that because the web service has a two way web method it is using a two way send port, for both these operations, the expected behaviour is for the SOAP adapter to realise one of the methods is a one way only call and to not expect, or return to the message box a response message. However, as the case appears to be, the SOAP adapter does seem to be returning a message to the message box even when no response was actually received, or indeed expected.