Hi Mark,
That is very strange and certainly not a problem I can reproduce. Can you post a link to the page where it happens?
Is it possible that the skin or page has an extra <form element that could be short circuiting the main form? Have you tried a different skin to see if the skin is a factor?
Are you using any custom form submission handler that might be deleting the submission?
Other than that all I can think to suggest is to try is redeploy the files (maybe both the latest form wizard files and the latest mojoportal files), export the form definition, delete the form wizard instance, create a new one and import the form definition.
Hope that helps,
Joe