Plex does not index albums where the first track has a different Artist from the main AlbumArtist

Plex Media Server Metadata & Adding Files
2

When you enable “prefer local metadata”, it is crucial that your folder organization is immaculate.

  1. verify that all tracks of this album have their AlbumArtist tags populated at all and with identical content
  2. all albums which are stored together inside an AlbumArtist folder (like below) must have identical content in their AlbumArtist meta tags.

So, at all times stick to this folder organization (that includes samplers and compilations!):

/Music
   /Various Artists
      /SamplerTitle1
         01 - Track Artist - Track Title.ext
         02 - Track Artist - Track Title.ext
         03 - Track Artist - Track Title.ext
         ...
      /SamplerTitle2
         01 - Track Artist - Track Title.ext
         02 - Track Artist - Track Title.ext
         03 - Track Artist - Track Title.ext
         ...
   ...
   /Chicago
      /The Best Of Chicago 40th Anniversary Edition
         01 - Chicago Transit Authority - Track Title.ext
         02 - Chicago - Track Title.ext
         03 - Chicago - Track Title.ext
         ...

If you edit AlbumArtist folders or generally all kinds of tags which should cause a structural change in your library (like the switch to a different Album Artist), you might have to perform a Plex Dance “light” for the change to take.
(“light” meaning you can omit step 4 of the dance)