Even with Prefer embedded metadata enabled, Plex will attempt to match your albums and to provide additional information (e.g. reviews for albums, artists… and album types).
If you don’t want your albums to be grouped by type, disable Settings > [Server Name] > Library >Group albums by type in your server’s settings.
There’s certain metadata editors that can write the proper ReleaseType tag to manually set a group (following the structure/convention of Musicbrainz). In addition there’s a feature suggestion asking for Plex to offer an option to manually override the album release type → Music Release Type (Category) - Manual Edit
