Phone is now en route back to the Czech Republic via DHL for a second repair... Having found an online video showing what's inside (and it matches the ROG 8 ) the fault is where the ribbon cable on the back cover connects with the main board via what look like two connectors that may be just squeezed against each other. In my situation the contact is bad and degrades with time, suggestive of a fractured ribbon cable, poor conduction, or a dry solder joint where the terminal block is attached to the circuit board. Given the same wires seem to control the LED and the NFC signals this explains why the light goes out when you tap the read cover in the middle, why the light doesn't always come on when you try and turn it on using software, and why the NFC fails. I have found applying pressure between front screen/back cover with finger/thumb over the area where the two square contacts are makes the NFC work every time and from a greater distance. The light also stays on if you squeeze in this area, and then tap the phone on the back, which would previously have made the light go out, or go to half brightness. Close examination of the black cover in this area shows a little movement is possible when squeezing, so the previous repair can't have sealed the case properly in this area, and no doubt left the phone susceptible to water damage had it got wet. An image from the video I found is below.
If you have a similar problem try squeezing the phone (between glass and back cover) a few centimetres below the larger camera lens and see if the problem temporarily goes away.
