[Les Kern]

Okay, I have verified this with multiple uploads with different numbers of books. Of course I have no idea why this happens, but happens it does.

I reached around 5,000 books and documents on my Oasis 32GB 10th Gen. I went to add 10 more books, selected ""Eject this Device" from the Calibre menu, no covers. DARN IT!
So what I finally tried was, that after the books are uploaded, I just closed Calibre, and used the computer to eject the Kindle.
Covers!
Woot!
Like I said, this happens EVERY time.
While I am thrilled, I thought I'd post this in case it happens to others so they can try this method, and perhaps Mr. Goyal will be interested.
Onward and upward,
Les