Draw the Plex Dance out to several days,
i.e. remove all the music from this artist,
then let your server perform its automated server maintenance (usually at night).
Since there are some cleanup operations which are only performed once a week, I cannot say withg certainty when the last remnants are cleared from the database. Hence my recommendation to draw it out to a week and only add the files back after that.
But before you do that, verify that your files have the correct name in the Album Artist meta tag.
This is different from the regular Artist tag. You cannot see it in the Windows file properties.
You’ll need a dedicated metadata editor, like e.g. mp3tag or Kid3 to edit this.
