Unless you re-enter the same key information for a work, whenever you edit a work that information is automatically deleted.
examples
Added a URL to work: (note: Only action I performed in the work editor was to paste the new URL in the URL section)
http://musicbrainz.org/edit/28263701
Correspoding deletion of key information:
http://musicbrainz.org/edit/28263700 or http://musicbrainz.org/edit/28263634 or http://musicbrainz.org/edit/28263532

also I can't seem to be able to re-add this information which is this problem here apparently:
http://tickets.musicbrainz.org/browse/MBS-7612 or http://musicbrainz.org/edit/28263633 or http://musicbrainz.org/edit/28263531