On my MU install adding a new site for the root domain works as expected.
I go to sites > add site and fill in the detail and the site is created in a moment.
I am using multi-domains as I want to offer my users options in domain creation.
When I try to add a new site with a non root domain I am referred to the wp-signup page and have to fill in all the data again.
The page is auto filled with the root domain as the new subdomain name for some reason.
I know that there is something wrong with how this is working and not sure where to start troubleshooting.