I don't see any obvious reason on why your parameters are not forwarded to the secured page. However I tried this scenario on my setup and here is what I found out.
If you are using self-signed certificate then Internet Explorer will show you warning page that it does not trust the certificate and this warning is shown before actually invoking your secure URL. At this point IE actually has lost all the parameters that you have passed in your POST request. Firefox seems to be smart since it reposts the request once you confirm the certificate warning message and hence it works on Firefox. But IE8 does not repost the request for some reason.
So once you confirm the certificate warning just hit refresh on your Flex application IE window again and it should work this time, test it.
I don't have any solution to this but workaround is to trust and install the certificate into your browser permanently and it should solve your problem for now.