Because you have activated “Prefer local metadata”.
Plex is only doing what it has been told.
Most important:
Verify that all your albums have a proper “Album Artist” meta tag. (That is a different tag than the regular “Artist” tag.)
All tracks on a particular album must have the same “Album Artist”.
All albums which have the same “Album Artist” can be stored together like this:
/Music <-- library root folder
/Various Artists <-- AlbumArtist folder
/Sampler 1 Title <-- album folder
01 - Artist - Track.mp3
02 - Artist - Track.mp3
03 - Artist - Track.mp3
...
/Sampler 2 Title <-- album folder
01 - Artist - Track.mp3
02 - Artist - Track.mp3
03 - Artist - Track.mp3
...
The opposite (storing albums with differing AlbumArtist tags [or no Album Artist]) inside the same artist folder must be avoided!
This is the crucial rule which, if not followed, can result in the mess that you are curently dealing with.
