DIAGNOSIS The 5-way joystick/navigation button is misaligned or has debris lodged around its base, preventing clean directional presses. This is a physical, not software, issue.
FIX Power off and unplug the monitor. Inspect the joystick. Use a can of compressed air to blow firmly into the gap around the joystick's base to dislodge any dust or debris. Do not insert any tools. After cleaning, plug the monitor back in. Test the joystick with deliberate, centered presses: push directly in for each cardinal direction (up, down, left, right) and straight in for 'select'.
IF THAT DIDN'T WORK The issue may be internal wear. You can temporarily bypass the joystick for most functions. Download and install the ASUS DisplayWidget Lite software for your computer. This allows you to control monitor settings like brightness, input, and ProArt presets directly from your desktop, using your mouse.
HEADS UP Avoid using excessive force on the joystick, as this can worsen internal damage. If the joystick fails completely, you will need to use the software or seek repair for physical button replacement, as there are no other physical controls on the monitor.