Possibly related to
https://forums.plex.tv/t/plex-doesnt-refresh-libraries/775772/
https://forums.plex.tv/t/library-updater-does-not-manually-update-library-media/772681/
Although since yours is specific to Music libraries it may not be.
People have reported that the last version that didn’t exhibit hangs is 1.25.3.5409, in case you want to roll back
