My monitor has a resolution of 1920x1200. I run the game in a new window and print out the screen resolution via the aforementioned C++ function. This prints out 1280x720. Then I do exactly as you describe in steps 1-5, except that I have set both the “Play in New Window” as well as the “Play in Standalone Game” resolutions to 1280x800. Then I press the “Set as Default” button and confirm everything.
However, when I run the game now either in a new editor window or as standalone game the resolution returned by the C++ function differs from the resolution set in the editor settings.
Can I provide any other information for you to track down this issue?