A GET https://musicbrainz.org/ws/2/artist/51823891-6035-4d6c-a007-b70a86747023?inc=url-rels, with Accept: application/json in the headers is returning an XML result.

The same request with a different inc value (e.g. inc=aliases) returns JSON just fine.

A request with inc=url-rels but a different artist ID (e.g. 51823891-6035-4d6c-a007-b70a86747023) also returns JSON.

Based a test in the browser using https://musicbrainz.org/ws/2/artist/51823891-6035-4d6c-a007-b70a86747023?inc=url-rels&fmt=json does return JSON, so it seems like some subset of requests is somehow losing the request for JSON (but only when requested via the Accept header, not via the fmt query parameter).