OK so I know this thread is old, but I
finally got around to following this up, instead of just making do with converting books to mobi to read on my kindle. It's just a few different things I tried in an effort to understand what the problem was.
tl:dr - well after all that, still don't know
exactly what is causing the problem, but there's an easy solution, see below.
Calibre Set-up:
Device details:
TEST 1.
TEST 2.
TEST 3.
TEST 4.
SOLUTION
The fix is easy enough, and is as Oibgzfka suggested.
Convert the downloaded azw3 to another format e.g. epub, then convert it back to azw3, & the book now shows up quickly & exactly as it is supposed to on the KV, with its cover intact.