After I updated the Kindle Scribe to 5.16.3, and connected to calibre (which unfortunately takes much longer now with a large library), azw3 files no longer display thumbnails of the book covers in Library view.
I've tried:
1. restarting the Kindle
2. converting to azw3 and checking the "Share with Facebook etc"
3. converting the jpg of the cover image into png
4. uploading the book and allowing the kindle to load the book, then reconnecting the kindle to calibre
5. opening the book and giving the kindle time to identify the cover
6. turning off wifi and assigning an undefined publication date.
The books display the cover on the sleep screen.
Edit: it seems that the connection to the Scribe from calibre is slowed down by all the ._ files that the mac produces. If they are removed by dot_clean or other method (such as
MobileFileSearch v1.46, getting the list of books on device through calibre from the reader is much quicker.