While I can't think of an example in the USA, another reason for cut off smokebox doors, and matching smokeboxes was streamlining of locomotives, where the smokebox was cut down to improve aerodynamics.
The first example of this was Nigel Gresley's W-1 4-6-4 No 10000, where to avoid a cut off door, a very small diameter door was fitted. However, I think there was a photo of the locomotive under construction showing a conventional smokebox with full size door projecting above the frames of the casing, where a cut down door might have been applied in the style of the A4, but was not.
Gresley's A4 Pacific had a very abbreviated door to allow the "Bugatti nose", although this was concealed by the streamlined casing and a cosmetic flat outer door. Willian Stanier's "Coronation" didn't need as much of a reduction in the smokebox, so it had a circular door but a sloping top to the smokebox.
A similar situation existed in Germany, where Richard Wagner's Class 05 had a cut down smokebox, and the feed water heater moved behind the stack. The later and more numerous 01-10 and 03-10 streamlined Pacifics had the feed water heater moved forward and slightly down to clear the casing, which became very obvious post WW II when the streamlining was removed. All of these that survived were eventually rebuilt with a conventional shaped smokebox door, in some cases concealing the feed water heater in the same location, or with different mixing-type feedwater heaters of different designs in East and West Germany.
But these were definitely examples where the door, as well as the smokebox was cut down.
You can't really mention this subject without including Oliver Bullied. His Q-1 heavy freight 0-6-0 had what is best described as "horseshoe shaped" smokebox and boiler cladding to allow use of insulation material that could not be contained under normal cladding (said to be a wartime exigency) but a circular smokebox door.
Bullied's Pacifics, both the Heavy "Merchant Navy" and the Light "West Country" had strangely box-shaped smokeboxes concealed by the angular streamlining and what is best described as a "parabolic" smokebox door.
To return to the USA, The Norfolk and Western Y6a and Y6b both had a severely cut back lower smokebox but a small circular door.