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Calibre is creating a correct image, but after ejecting the kindle and re-plugging-in, the same thumbnail file has been replaced by the 'no image found' amazon thumbnail.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 kindle 2020