Sanguine I really don't know...
My thoughts are as follow: the forum is correctly configured if a visitor can't see any security error at any point.
A fourth-level www domain not resolving is not a concrete error and the browser simply will tell the visitor the address is incorrect. No harm done.
However if the domain does resolve it becomes something the user might type and should not show a security warning.
If the domain uses HSTS with includeSubdomains and/or preload and types a fourth-level www subdomain it will automatically load the HTTPS version of that page and cause security warnings, so redirect when http is not enough.
What about not answering on port 443 for that subdomain ? That would trigger a proper not found message in the browser and that's something I could say is acceptable in my rating. But if it's the same IP answering all urls you have to wait for the Server Name Indication and this will probably be too late to make it look like port 443 is closed.
Damn that's not easy...