Input validation not returning errors

I am currently developing a flow in webmethods that I am exposing as a webservice through a provider.
This flow has an input document and no output defined and I have selected to validate the input. When I omit a required parameter from the input document I get no errors returned. (for example in soapUI, I get an empty reponse)
But, I see that there are input validation errors in the server log even though it is not returned to the output.

Here is the strange thing. If I add an output parameter to the flow (a simple string for example) then the validation errors are returned immediately.
Is this a know bug? Or is this a feature? Am I missing something here?

what fix levels is your IS carries ?