Don't know if it's just me, but ...
Disappearing covers problems was solved 4.18.
Just tried adding a new, sideloaded book, and the cover disappeared. Confidently, I plugged back in the Kindle and let Calibre (4.19) do it's magic.
Cover reappeared and then disappeared again
Have tried this on two kindles and from two laptops (both Mac). Any new book I add now the Kindle is replacing the cover with the 'default' one.