Thanks for the suggestions GRinRI and Grizzly. I called Hitzer today and they said:
(1) Make sure I didn't bend the hinge when installing the door. Since you have to lift the door and maneuver your hand inside to put the nut on the screw, it is easy to bend the hinge because the door provides a lot of leverage. I checked mine and that's not the problem.
(2) Sometimes the hinge can sit slightly too far back, toward the stove body. Then the top of the door can contact the edge of the heavy steel intake tube and get hung up. That DID appear to be my problem. Hitzer's solution is to ream out the bolt hole in the hinge, making it slightly larger, so the hinge can sit a little more forward away from the stove body. I didn't have to go quite that far. The hole in the hinge and/or the intake tube already was very slightly oversize compared to the bolt. I put the tip of a screwdriver between stove body and hinge and twisted to force the hinge forward. Just a tiny fraction of an inch, but I think it has fixed the problem. I can't get it to stick open like I could before.
Like I said, this was a retrofit of Hitzer parts to my Harman stove. It's quite a contrast that Harman won't even talk to a stove owner, but Hitzer instantly put me in touch with someone who knew all about the various generations of hinges and intake doors and the occasional problems associated with them.