Does adding the alias manually from the Administration work as you expect?
Do you also set the redirect behavior?
Or have you set the site wide setting:> "Use site settings (default) - with this option, the redirection behavior follows the Redirect page aliases to main URL setting specified for the website in Settings -> URLs and SEO."
Verify you use the correct culture for your alias in code... the
.Culture("en-us") part for the document selection from the sample code.