Initially, gender name was lower cased in the output of MusicBrainz web service.

Since 2011, Lookup & Browse requests to MusicBrainz web service are returning artist gender name in the same way it is displayed on the website: in sentence case where the first word is capitalized. (See commit 98df86b9 to musicbrainz-server.)

Not sure how search was handled at that time, but when support for artist has been added to search in web service version 2, it was still lower cased, most probably because the development was long and the code has not been changed accordingly before release time. (See commit e6bb09a1 to archived search-server).

The development of the Solr-based search reproduced this behavior faithfully in both the search schema (see commit 7688c955 to mbsssss) and the indexer (see commit d326c98 to sir / ticket SOLR-55).

Search requests should return gender in the same way as the Lookup and Browse results: by not lower casing gender name.

That requires to stop the indexer (sir) from lower casing gender name. Instead, the schema (mbsssss) should lower case both index and query for analysis.