What I had learnt from investigating this issue is:

* It's been a problem since at least 2012

* HDMI cables have an active-low connection pin, therefore it's easier to fool the video card into thinking the monitor is still connected.

* DisplayPort is an active-high signal, therefore if the monitor powers off, it's the same thing as the cable being physically disconnected.

Ideally I would be able to switch the monitor off in such a way that it still maintains a connection, just that the display is off.

Edit: formatting

Is it necessary to you that the display actually be disconnected from power? I use display port on most of my displays and don't have the issue with simply turning off the monitor. But, that's because the monitor doesn't REALLY turn off. It just goes into an ultra low power mode. If that satisfies you, perhaps you need to get a different monitor is all.

Perhaps. I should take a look at the monitor configuration and see if there's a setting I can change. Something like "monitor power button puts monitor to sleep" rather than "monitor power button turns power off to monitor".

I don't turn the power off at the source, i.e. unplug the monitors.

Sounds perfectly normal to me. I’ve only ever seen the behavior daemin describes.

See my reply to the subthread commentor.