I knew I had dealt with this problem before without resolution:
I use Picard to tag my MP3’s, and if you check with ffmpeg or ffprobe, Picard stores the tag as publisher not label. And, since Plex uses ffmpeg to find the tags, it fails, and the Record Label is grabbed from MusicBrainz.
Input #0, mp3, from '01 - Right Beside You.mp3':
Metadata:
title : Right Beside You
artist : Sophie B. Hawkins
track : 1/11
album : Whaler
disc : 1/1
date : 1994-07-26
genre : Rock
TMED : CD
TLEN : 286626
publisher : Columbia
TDOR : 1994-07-22
SCRIPT : Latn
TSRC : USSM19400973
ASIN : B0000028XW
TEXT : Sophie B. Hawkins
album_artist : Sophie B. Hawkins
Maybe we can get the new version of ffmpeg included in the test version just released for multiple genre tags in ID3v2.4 to map publisher to label for MP3s