This is similar with any other desktop system with a separate display. The AIO is just the system and display built into the same housing.
However, with traditional system, when the system is set to turn the monitor off, it will simply turn off the signal from the video output in which the monitor will detect no input and will display similar "No input" message. The difference however is that the display will normally turn off after a few seconds. In the case of the Dell, it did not.
It is clearly a design flaw which can only be solved with a new firmware for the display and not the system.