Using the "Format performer" tag loading e.g. https://musicbrainz.org/recording/e55e5bca-7fe2-436b-9afa-a5338fae32b6 the "lead vocal" relationship gets truend into two relationships, one for "lead" and one for "vocals". Same happens for "background vocals". I used the standard settings of the plugin
@rdswift Can you take a look?