When you edit your Library, on the Advanced tab, at the bottom of the page, is a setting:
“Plex Music Only” should force Plex to only look online (for album art only. I don’t think this affects Artist posters). “Both Plex Music and Local Files” would prefer local files, if found, but go online if it finds nothing local.
If Plex wasn’t configured to ‘prefer local metadata,’ there are several things at play. The folder/filename structure is important, but it’s also important that the online databases (MusicBrainz, primarily) have accurate information.
There’s more to configuring Plex for local metadata than that one checkbox.
