[DNSB]

The cover thumbnail bug is not fixable in the MTP Kindles since the folder the cover thumbnails were copied to is no longer accessible (the entire system folder is not exposed over MTP).

You can work around by changing the cdetype of the ebooks you send to PDOC or using Send to Kindle which also sends PDOCs. The KFX Output plugin allows using PDOC instead of EBOK (check the Create personal document instead of book option). Converting to MOBI also allows using PDOC but for azw3/KF8, you will need an external utility to change the cdetype to PDOC.

Edit: You can select Enable sharing of book content via Facebook, etc. and that will generate a KF*/azw3 file with the PDOC type.