Question for @OTA and @NightWatcherTheater. After scraping all movies in my library to store the nfo and fanart in the movie's directory I then downloaded a custom movie poster for a few titles (didn't like any of the options available in TMM). Once completed, I went to Kodi and scraped all movies using local info only. Most titles displayed everything correctly -- posters, fanart, clearlogo -- but Kodi did not display some of the custom movie posters that I downloaded and told TMM to use. The filenames for the posters are perfectly fine in the movie folder's directory (movie_name-poster.jpg) and I can manually select the custom poster after browsing to the Item Folder. Have you come across this issue? (Note: This only seems to be affecting Kodi installed on my Firestick 4K, not my nVidia Shield Pro).
Figured it out: The Firestick is accessing my media server via FTP (faster) whereas the ethernet-connected nVidia Shield Pro is using the SMB server. This is an odd bug, I'll notify the Kodi developers.