With the new Ubuntu Autoinstall mechanism, one cannot create a root user during provisioning (non-root user work fine).
The provisioning works, but the root user is not enabled afterwards neither is the given password set. Only the ssh key is installed correctly, so one could login using ssl authentication.

As a Foreman user, I would expect the root user to be available after installation (if that's the default user). Accordingly, the provisioning template must be adapted to actually create the user as it works for non-root user.