Okay, I FINALLY figured this out. The reason this was not working was due to Google Drive backing up my Calibre Library folder, and when Drive accessed the files, some would through and permissions error and some wouldn't, allowing SOME covers to be copied to the Amazon Cover Bug folder.
The Calibre book files were being routed to a standard library folder in my documents folder called "Calibre Library." However, I recently selected that folder as one of the folders for Google Drive desktop app to scan and upload. This background usage/scanning of the folder threw permissions errors and resulted in
very sporadic behavior.
Another clue was that when deleting books from my library via the Calibre app, the app would sometimes throw an error that the file was in use by another program, although I had no knowledge of Google Drive performing a scan.
I paused all syncing and removed the Calibre Library folder from the backup options in Drive, and afterwards everything worked perfectly.
The sporadic behavior initially made me look for differences in the book files themselves, then explore permissions issues before FINALLY noticing today that the Cablibre Library folder has recently been added to the backup schedule. Don't make the mistake I did everyone- backup and then
turn off automatic syncing!
Thank you Kovid, it was your advice about debug mode that finally lead me down the right path!
