Any of the newer Kindles that use a MTP connection do not expose the system folder which makes it impossible to update the thumbnails which is how calibre worked around the original Amazon cover bug. If you send the ebooks with a PDOC cdetype, you should see the covers. However calibre will currently do this for mobi and the KFX output plugin will do this for KFX output however KF8/azw3 will require you to run a Python script to modify the files.
One friend of mine is currently using calibre to save his azw3 files to a single folder, running the script and then copying them to his PW 12th gen.
You can find the script on GitHub at:
ebook_cdetype_to_pdoc