[gibxxi]

To get individual "per-disc" artwork to work in Kodi (in both browse and playback scenarios) each disc's music tracks needs it's own unique "Disc X" folder underneath the parent "album" folder, and the unique "folder.jpg" and "cdart.png" files for that disc stored in those sub-folders. Believe-you-me, I've spent more hours fighting with this (and various apps doing their best to foil my efforts) than I care to mention.

The good news is, if you use cdART Manager, it will correctly download (assign, and locate) multi-disc images when the above is true, assuming that multi-disc artwork for that album has been uploaded to Fanart.tv, and isn't below any rating cutoff you may have stipulated.