Including the disc number in the filenames cause problems for Plex. Even when using “Prefer local metadata,” Plex still depends on the folder/file structure. It’s important to follow the naming convention outlined in this Help file:
https://support.plex.tv/articles/200265296-adding-music-media-from-folders/
With multi-disc albums, it is especially important that the discnumber tag is correct. This How-to article may offer additional help. Pay particular attention to the section on multi-disc sets.
