One of the problems with sideloading .mobi files is the PDOC tag. There is a discussion on the problem and fixes here:
https://www.mobileread.com/forums/sho...d.php?t=162642
but the short answer is to open Calibre. Click on Preferences -> Conversion Output Options -> MOBI Output -> Under "Kindle Options" remove the PDOC tag and Apply.
Now reconvert your books mobi-mobi and they should appear in the correct place under Books.