Screen blinking "HDMI no signal" when Mac is sleeping I usually leave my computer, and let it go to sleep after 15 minutes.

My external screen is showing a blue screen saying "HDMI no signal" every other minute while my MacBook is asleep, and the screen lasts for a few seconds. If I unplug the external screen, the message appears for a longer period of time.
It did not happen with other computers (I have tested with a MacBook Air and a Windows Surface PC).
The built-in screen is turned off and does not blink in any way.



Setup:
I have a MacBook (Retina, 12-inch, 2017) running MacOS Mojave (10.14.1). I have a USB-C Digital AV Multiport Adapter, to which my external monitor (ASUS VS248H) is connected using a HDMI cable.

Question: Is there any way to avoid screen blinking (except for turning the screen off when I leave it)?
I.e. sending the correct signal to the external screen, so it does not think it is disconnected.
The Apple Digital AV Multiport Adapter is a passive converter meaning that it's just "rearranging" the pin outs from the DisplayPort signal to the HDMI signal - basically, the monitor is detecting voltage, but there's no data being sent.

I expand on this more in this answer to a similar question.

Instead, you should be using either of the following with your monitor which supports DVI and HDMI:


Type C DisplayPort to DVI adapter (active is "better", but passive is OK here since DVI and DisplayPort are similar signals.
Tyep C DisplayPort to HDMI active adapter.