When I converted books from one format to another, the covers were not displayed on my KindlePaperwhite6 (12th Gen). For myself, through trial and error, I found a simple solution: after converting the book, I changed the extension to MOBI. Voila - the covers are displayed.
I did the conversion with the
freeLib program (
https://sourceforge.net/projects/freelibdesign/). I use Linux version 6.1. Files converted by Calibre sometimes do not open.