That is a bug in Amazon's Personal Documents service. In order to understand what is going on, you will first have to understand a bit more about covers.
For books you download from Amazon (or actually anything that has an ASIN), the Kindle will ignore the cover inside the book, and attempt to download the cover from Amazon's server (using the ASIN to identify the correct book to retrieve a cover for).
For sideloaded books, assuming you do not use calibre or Java Mobi Metadata Editor to embed an ASIN, the cover is extracted from the ebook file.
All this is irrespective of whether or not there is an actual, honest-to-goodness cover embedded in the book.
If you sideload with calibre, caliibre will notice that the book has an ASIN, and sideload a cover to go with the book -- this is
very useful when the ASIN is a fake one generated by calibre's conversion
because otherwise you would have *no* cover.
Why does this matter? Because a book that has gone through the Personal Documents service has an ASIN -- the one that links it to your cloud account.
The cover will
not be extracted from the book.
And there is apparently a bug in the service, that prevents it sending covers for an AZW3 (but a MOBI works fine).
@quiris has written a cross-platform application that will extract the cover from the *.azw3 and generate a cover thuumbnail, see post #14 of this thread:
https://www.mobileread.com/forums/sho...d.php?t=212242