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Because the Kindle only extracts thumbnails for books that show up as a "Personal Document".
Otherwise, it expects the book to be downloaded from Amazon's servers, which means it ignores the embedded metadata cover and attempts to use the exth 113 (ASIN) metadata field locate the ebook via Product ID on Amazon's servers, and download a cover thumbnail from there.
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When using calibre's Send-to-Device option, calibre will sideload a cover thumbnail together with the ebook itself.
But books downloaded via the builtin browser or sideloaded manually via USB though copy-paste in the File Browser do not enjoy that workaround, so this Kindle bug strikes.
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