Elitebook 840 G8 - not charging on rear USB-C port

[IainHu]

My Elitebook G8 has suddenly started to not charge via the rear of the two USB-C ports on the RHS. 

The laptop is plugged into a Philips screen providing power via the USB-C. It has been working like this for maybe a year without issue. This morning the machine was acting very oddly and it turned out to have no power, despite being plugged into the USB-C. It hadn't been moved or disconnected, so all good for months, hibernated overnight, USB-C in, this morning all over the place due to 0-2% battry charge meaning it has discharged overnight despite being hibernated.

Working through the issue:

1. The power from the screen and USB lead is fine and now proven by all being well when plugged into the forward of the two USB-C ports
2. There were a number of HP updates last night, but these are keyboard (11.1.9.1), "SoftwareComponent" 8.10.29.1 which i believe is hotkeys, and "SoftwareComponent" 1.82.4285.0 which seems to be speaker and mic. Nothing suggesting power or USB. There was also, however, an Intel display update 32.1.101.6651 and the display is of course connected via USB-C, although this is meant to be a video driver...?
3. To clarify, I now discover the same cable from the same screen powers and charges the battery fine via the front port, but when in the rear port, despite the charge icon coming on and the charge LED by the bullet power supply port being on (white), the machine loses charge. It looks as if it is on on charge, including disconnecting the bullet power supply if also inserted, but isn't actually charging

There is a similar thread about this at: https://h30434.www3.hp.com/t5/Notebook-Hardware-and-Upgrade-Questions/Elitebook-840-G8-USB-C-chargin...  This includes a power-down, holding the power button down for 30s or more, then releasing AND DOING NOTHING reset of the machine. The machines goes through a few cycles of keyboard LEDs coming on and going off, the cpu fan spinning up, shutting down, starting up again and finally restarts for real. This is offered as the reset solution to cure this apparant ill, only it's not curing my issue with the rear USB-C port. 

In summary, despite attempted resets, I am left with front port good, back port bad, despite 'back port' having been in use and working perfectly for months. This means I have a workaround (use the front USB-C), but still having 'something odd' going on with the rear port, and for no obvious reason