I was stuck on this same problem.
Domain forwarding was working for all pages, but it was not working for the naked root (i.e. forwarding wasn't working for "www.mydomain.com" )
I was able to resolve the issue by clicking the "Add Preset" button shown in the screenshot below, and then selecting "Squarespace Defaults" from the list of options to add. Now all pages as well as the naked root domain properly forward where they are supposed to.
Yes, "Squarespace Defaults" is both the title of the table and also the title of the option you need to select after clicking the "Add Preset" button.
