external secure url in form action changed when "Switch back to http pages when there are no matches" is set

I have a form that submits the POST data to an external URL lets call this https://www.external.com/purchase

If I have "Switch back to http pages when there are no matches" checked, the URL in the form is changed to http://www.mydomain.com/purchase

All other aspects of securepages is working fine and as expected, this is the only anomaly from the module.

For the time being I have unchecked "Switch back to http pages when there are no matches" which makes more pages secure than I would like to be.