First, thank you for Quality Check Plugin!
When I apply "Fix ASIN for KindleFire" to an AZW3 then transfer it directly to my Kindle using USB (not via Calibre or Send-to-Kindle), my Kindle will download a cover from Amazon's servers
However, this does not work when I do the same with a dual-MOBI

When I apply "Fix ASIN for KindleFire" to a dual-MOBI then transfer it directly to my Kindle, it will open as an AZW3 rather than MOBI, but I get a generic cover.
Is this just the way it is with MOBIs, or am I missing something? Thanks!