And because it is smaller than the smallest allowed cover image size a generic cover thumbnail is instead shown for that book on the Kindle. This overwrites whatever image thumbnail was set by calibre. It appears that Amazon's servers have just been changed to return a 60x40 pixel image containing the text 'No image available' instead of nothing. In the past the request for the thumbnail image would fail because the ASIN is not a valid one at Amazon. It does this whether or not a thumbnail for that book is already present. When a new book is found in storage that is marked as an Amazon-sourced book (EBOK) the Kindle software initiates the download of the cover thumbnail for that book based on its ASIN. There is a thread on Reddit about it: All my book covers just disappeared?!

The same thing is happening for AZW3 files. I can work around by replacing the 'no image found' thumbnail with the cover art (using save cover to disk) on re-plugging-in, but I need to note which thumbnail goes with which book before I eject! Just checked, when Airplane mode is on, the cover art remains as soon as airplane mode is turned off, the cover art disappears. If I understand correctly, normally the kindle asks amazon servers for artwork for a sideloaded book, so presumably it is now doing this regardless of the presence of art in the thumbnails folder? Looking at the system/thumbnails directory, Calibre is creating a correct image, but after ejecting the kindle and re-plugging-in, the same thumbnail file has been replaced by the 'no image found' amazon thumbnail.

The last time I successfully added a book to the Paperwhite must have been within the last two months, has there been a kindle software update since then? The kfx files had previously been uploaded to my old Paperwhite (2018) with working cover art thumbnails but I tested again today and it's now showing the same behaviour as the Oasis. The cover art thumbnails on the home screen briefly appeared correctly but almost instantly vanished and replaced by the default blank cover. Uploaded my kfx files (converted epubs) with Calibre send-to-device to my new Oasis (2019).

Calibre android kindle