It is a strange issue.
Even if the cover is identified, it doesn't show up as thumbnail.
The same thing happens if you sideload a KF8 in Kindle Paperwhite.
Code:
<meta content="cover.jpg" name="cover" />
No problem at all if you sideload the ebook as epub.
I mean I don't connect the device through USB. I use Dropbox to download the ebook using the built in web browser...