Thanks to both.
It was the MOBI/AZW3 Output preference which had not been set to use author sort for author.
After setting that, I reloaded the books, and all is fine.
That leaves me with the question of how it came to be unset in the first place. I have been using my Kinde in conjunction with Calibre for some years now.
That will remain one of life's little mysteries.
Thanks again.