All is well that ends well.
Just for the record, as often happens, a "key" piece of info was missing initially (the fact that it was an "install from USB"), and I guess that having posted it here:
Unattended Windows Discussion & Support
Member Contributed Projects
Install Windows from USB
sent people looking for a wild goose chase, at least that happened to me , I was completely deceived by this issue that while it didn't at all look like a X,Y,Z problem, was actually a (slight) form of the X,Y,Z problem: