Wow, you already made an update! Tested buttons on end screen and they work!
But, it looks like keep_height alone isn't enough. You should do next:
1. In Godot 4, toggle on advanced settings (upper-right corner of the project settings window);
2. Make your "Viewport Width" and "Viewport Height" same (if your game resolution was 1280x720, make Width be 720 too);
3. Make your "Window Width Override" and "Window Width Override" have same values like "Width" and "Height" in the beginning (1280x720).
This should allow you to resize width while keeping height. Additionally you can toggle during test "Resizable", click on Play and try to drag window to see if this works then turn it of after test.
Thank you for your effort, hope this works out!