Can you give an example of a page where this happens? If you use relative URLs in the HTML the browser will construct the full URL based on the HTTP location.
In both of them, the action link for register is /mpv/register. The request processor is going to redirect the request from /mpv/register to /register if you use host-node mapped URL, but the POST will fail because the bug I've mentioned above.
Right now the only way it works is because this patch I've supplied.