We've updated the site forwarding behavior to allow redirection of deep URLs for a given domain name. So if a user types a long URL (or bookmark), and the site is, for example, forwarded because of a maintenance window, the user will be redirected to the maintenance page instead of getting a 404 error (from our servers, not the website). This does not create any compatibility issues as the previous behavior is unlikely to be the expected behavior.
This behavior is the same regardless of whether the site is just forwarded, or forwarded and masked. If this causes any unexpected errors with your site, please contact support.