This is what the previous release editor did, even though the field was documented on the seeding wiki page. Now that the field actually does something, userscripts that have been setting it incorrectly now break.

It also seems more intuitive to derive the position from the CGI field names, though the previous RE didn't do that either (it just reset everything to sequential order: https://github.com/metabrainz/musicbrainz-server/blob/die-spammer-die/lib/MusicBrainz/Server/Wizard/ReleaseEditor.pm#L1663).