With SSO, Lithium redirects the user to an external URL (configured in Lithium admin). Lithium includes the URL of the original page (in the parameter return_url) so that the extenal site can redirect the user after they have logged in. However, the implementation of this is down to the external site.
It's hard to know for certain without seeing the site, but I suspect that at some point during the redirection, the extra parameters are getting stripped out of the URL (probably when the external site redirects the user back to the community).
If the user needs to be logged in to access the page with the iframe, it might be worth exploring the option of retrieving the email address from FreeMarker rather than the URL.