Metadata should go into the embedded meta tags of your files. Plex will not touch these, ever.
In order to have Plex actually obey these embedded meta tags, you need to enable “prefer local metatags” in the properties of your Plex music library.
But if you do that, you better ensure that all your music is tagged correctly to a minimum.
Here is a list of other threads where the MO is explained further: Album artist incorrectly set as AC/DC for thousands of tracks - #3 by OttoKerner
