DIAGNOSIS The monitor is receiving a false wake signal via the HDMI/DisplayPort cable. This is often caused by a feature called HDMI-CEC or DisplayPort wake signal sensitivity.
FIX Disable any 'HDMI Control' or 'CEC' setting in the monitor's OSD menu, usually under 'System' or 'Others'. If no such setting exists, the fix is at the source. On your PC, go to Device Manager > Monitors > Generic PnP Monitor > Properties > Power Management. Uncheck 'Allow this device to wake the computer'. This also often prevents the monitor from self-waking.
IF THAT DIDN'T WORK A cheap or damaged cable can leak small voltages that trick the monitor into waking. Try a different, shielded DisplayPort or HDMI cable as a test.
HEADS UP If you use a desktop with a dedicated GPU and integrated graphics, ensure the monitor is plugged only into the GPU. Mixed connections can cause weird power state signals.