Screen blinking "HDMI no signal" when Mac is sleeping
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.