DIAGNOSIS The monitor is not receiving a proper wake signal from the computer. This is often a handshake issue with the USB-C connection or a firmware bug.
FIX First, disable the monitor's deep sleep function. Press the menu button, go to Others > USB-C / USB-B Wake Up, and set it to 'On'. Then, go to Others > Deep Sleep and set it to 'Off'.
IF THAT DIDN'T WORK The issue may be with your computer's USB selective suspend setting. On Windows, search for 'Edit power plan' > Change advanced power settings > USB settings > USB selective suspend setting and set it to 'Disabled'. Test again.
HEADS UP If you use the USB-B upstream cable, ensure it's connected to a main USB port on your PC, not a hub. Some motherboards do not provide wake signals through all ports.