My Kindle won't show new sideloaded books in the Books library anymore.

I've always used Calibre to convert my .epub and .mobi books that I get from non-Amazon sources (such as direct publisher downloads or Humble Bundle) to .azw3 so that they will show up in my Books library with all the Amazon ones. When I went looking for one I'd recently converted and transferred it wasn't there.

Looking at the "date modified" on the files in my Internal Storage>Books folder it looks like anything 3/14/19 or older is still showing up in my library. Everything from 5/22/19 and newer shows up in the folder but the book isn't in the library. They are on my Fire as documents, but as a visual person I REALLY REALLY REALLY want need cover pictures in my main library!
I've updated my Calibre to the latest version. I've tried converting to .mobi with the [EBOK] tag instead of [PDOC]. I've tried downloading new copies. I've even downloaded .epub and converted to both .mobi & .azw3 to see if either of those would work.

Help!
I have a 10" Fire running OS 5.6.4.0 (636558520) and it has no updates currently.