As a test I tagged the album Top Down Convertible Sounds with Various Artists and refreshed the metadata. Doing that by itself does not make it show up in Compilations.
This is how they are stored and tagged.
Performing the Plex Dance on Jukebox Gems did not fix the issue. As soon as I moved the album back to the Various Artists folder and scanned the library, it did not tag it under Various Artists, and instead gave it a different random album artist.
Looking further for any differences between the “good” and “bad” albums, I see that Jukebox Gems has some ID3v1 tags for some reason, but Save the Last Dance For Me only has ID3v2.3 tags. Is this an issue where the library scanner gets confused by v1 tags?