27inch iMac display issue

Has anyone ever experience similar display issues on their Mac?

This is a 2018 27inch iMac, perfect physical condition. No spills or water damage. It tends to appear when the machine has been left on for a while (ie. overnight) and is in sleep mode. The screen will stay like this until woken up again. There are no other apparent adverse effects.

I have taken the machine into our local Apple authorised service provider (no Apple stores in New Zealand!!) and they could not find any fault.

Anyone have any ideas??