Thank you.
Taking your suggestion - it occurred to me that i was using the case USB ports, so I did try using the same cable via a motherboard USB port instead.
I saw the same behaviour:
Connecting to Win10 desktop via MTP Media Transfer mode generates a generic Windows device connection “ping”, followed by a notification pop-up from Windows Explorer saying “USB device not recognised”
When connecting my XA2 Plus (also running 4.4.0.68) via MTP Media Transfer mode it connects with no issues. Same USB port, same USB cable.
I also tried a USB TypeC port on the back of my motherboard and got slightly different behaviour:
Connecting to Win10 desktop the phone does not present the choice of Media Transfer (MTP) / Charging only, instead i get a little notification at the top of the screen that the device is now charging.
When connecting my XA2 Plus (also running 4.4.0.68) via MTP Media Transfer mode it connects with no issues. Same USB port, same USB cable.
I then tried using the original cable on another Windows PC, and it worked:
When connecting my 10 iii via MTP Media Transfer mode it connects with no issues. We see the expected behaviour in the OP.
When connecting my XA2 Plus (also running 4.4.0.68) via MTP Media Transfer mode it connects with no issues. Same USB port, same USB cable.
So, it’s something about this desktop (hardware/software), with this device (my new 10 iii). Which brings me back to my question:
Q - might this have something to do with the fastboot driver installed as part of the flashing process?
i.e. It is no longer the device Windows recognised it to be before flashing.