Diagnosis
The 5-way joystick is misaligned or has debris inside, causing it to register false inputs or stick in a depressed state.
How to Fix It
Power off the monitor. Use a can of compressed air to blow forcefully into the gap around the base of the joystick. While powered off, firmly press the joystick in all directions (up, down, left, right, press) multiple times to work out any obstruction. Power on and test.
If That Didn't Work
The joystick mechanism is likely physically defective. Contact ASUS support for a warranty claim. As a temporary workaround, you can control all monitor settings via the ASUS DisplayWidget Lite software using your mouse.
Heads Up
Avoid applying excessive lateral force when pressing the joystick inward. It is designed for light directional nudges. Pressing it sideways can bend the internal mechanism, causing permanent damage.