Unlike wooden doors, flush mounting a mortise on an aluminum door takes some extra work. You need to attach 2 ALUMINIUM DOOR MOUNTING TABS to the top and bottom of the inside of the mortise cavity and the mortise will be screwed into the tabs, so it sits flush.

Alternatively, you have the option to surface mount the mortise, eliminating the need for the mounting tab.

Aluminium mounting tabs

Flush mounted mortise

Surface mounted mortise