A.

This phenomenon occurs occasionally in devices using Windows operating system, but there are cases where the set resolution is reflected only in the UI.

In this case,
Instead of changing in “Display Settings” that appears when right-clicking on the desktop, try changing the resolution in the graphics card settings. (Example: “NVIDIA Control Panel”, “Intel R Graphics Settings”)
When changing, please change to 30Hz for UHD (3840×2160) and 60Hz for FHD (1920×1080).

Detailed description:
When changing in window display settings: The actual video signal output is UHD, but only the UI is changed to FHD and output
When changing in the graphic card settings: the actual video signal is changed from UHD to FHD