For anyone who might be interested, I think I've solved this problem.
When I changed the motherboard and chip I had problems I couldn't solve, so I took it to a local PC shop. The guy there couldn't solve it either, and so he took it upon himself to do a clean re-install of Windows 10, without asking first. I've just discovered that, for some reason I can only guess at, he allowed Tablet Mode - which I wasn't even aware existed.
It's strange because the PC wasn't actually in Tablet Mode, but the Tablet Mode switch was set to ON, "when I sign in" was set to "keep the mode I last used", and I can't recall now what "when I use this device as a tablet" was set to.
At any rate I switched Tablet Mode OFF, put "When I sign in" to "Never use tablet mode", and that seems to have solved the issue.
Curious.