Powder coat adhesion to the rest of the door and the aluminum is good. It is a poor combination to treat at the same time. Steel cleans better at a higher ph and a higher temp. Drop it down to aluminum values and the steel sometimes does not get clean. Steel and aluminum together forms a wonderfull battery.
Most chemistry is trading ions or electrons.
There is a very narrow range of your cleaner and phosphate that will suffice for both. I suspect that the aluminum parts are galvanically protecting the nearby steel surfaces from reacting properly with the acid processes in the surface preparation. The etching reaction that you want to occur with the steel is occurring preferentially at the aluminum surface, while the cathodic reaction is occurring exclusively the steel. That is why the surface of the steel around the hinge remains bright. If the steel is bright, the surface condition that you want for adhesion has not been obtained.