@malinowski - inadvertently found the solution!
After downloading latest .iso and copying it with Rufus with its default “recommended”, had the same problem as initially posted here.
Then tried again with Rufus as per @malinowski 's DD suggestion and KDE Neon installed as expected.
Perhaps this was the solution right from the get-go?
If so, why did it always work for me before with Rufus’ default “recommended” .iso?