Plex overhauled the Plex Music scanning agent several months ago. It is much more powerful, and more accurate, as long as your embedded tags are correct and your folder/file structure meets Plex’s needs. I thought my tags were in good shape too, but the new scanner revealed a fair amount of mistakes. I recommend double-checking your tags, and your configuration of Plex. Start with a multi-disc album that isn’t getting added correctly. It might be helpful to provide screenshots of the tags for the album, and the folder structure for the files.
