On NGS Select release groups for Johnny Cash it lists http://test.musicbrainz.org/release-group/50031fc5-b480-368a-a7f2-a5db73fd3370, then switch to 'Show various artist release groups.' and it is still listed, this is a bug. Its caused by find_by_track_artist in http://git.musicbrainz.org/gitweb/?p=musicbrainz-server.git;a=blob;f=lib/MusicBrainz/Server/Data/ReleaseGroup.pm, for this particular release group the join to artist_name_credit will cause it to get two rows (one for Johnny Cash and one for June Carter) and because June Carter doesn't match the artist you are listing (Johnny Cash) it will be returned, but it will also be returned by find_by_artist because Johny Cash is the matching artist so it appears in both lists.

find_by_track_artist appears to be trying to look for releases that contain a track on the release where one of the artists on the track is the artist but where the artist isn't one of the artists credited on the release group itself

The problem also exists when listing releases as well