If your COS Website is set up using SSL (HTTPS), you may notice that the apex (root or non-www) domain does not load when you directly enter it (e.g. https://mydomain.com) into your browser's URL bar. This is because, technically, the root domain is not certified or connected to HubSpot; it is connected via your redirection service provider to redirect to HubSpot, so the domains are likely hosted as such:
http://mydomain.com (pointing to a redirect IP) > 301 redirects to > http://www.mydomain.com (hosted on HubSpot)
If you wish to have HTTPS support for your apex domain, you will need to set that up with a service provider that offers HTTPS redirection services. For more information on the technical requirements for connecting a domain to HubSpot, please check out this article.