Ahah! The Plex Dance fixed it, but not right away. At first, it semi-fixed it, segregating the .ogg files into their own artist, but they now all showed up under a single album, sans track 9, the only .m4a file which was in its own album under the original artist.
So since you noticed these are Opus, I googled and found .opus is the recommended extension for Opus files. I renamed *.ogg to *.opus, repeated the Plex Dance, and now everything is as it should be. The .opus files are no longer segregated from the rest.
There is still this oddity, the lone .m4a file has artist suffixed to it, but it’s not a problem at all since everything works properly.
