By default MusicBrainz Server is using "/basic" request handler if the query is a single word, "/select}" otherwise. The only difference is that the former blanks the boosts from phrase field "{{pf" parameter which is otherwise defined for each core.

References:

• https://github.com/metabrainz/musicbrainz-server/blob/v-2023-01-10-hotfixes/lib/MusicBrainz/Server/Data/Search.pm#L814-L824
• https://github.com/metabrainz/mbsssss/blob/v-2021-05-14/common/requestHandler-basic.xml#L5
• https://github.com/metabrainz/mbsssss/blob/v-2021-05-14/common/requestHandler-select.xml
• https://github.com/metabrainz/mbsssss/blob/v-2021-05-14/area/conf/request-params.xml#L5

The above referred code has a comment which assumed that it was a bug in Solr. The linked bug report has been immediately closed as inappropriate and not further discussed on any mailing list as far as recent searches shown.

Reference:

• https://issues.apache.org/jira/browse/SOLR-12409

It should be investigated whether this behavior is expected or not, whether the endpoint and/or parameter(s) should be changed or not, and for which reason.