Currently, the plugin marc21_field_005 and its unimarc opponent fill field 005 when this field gets focus. This implies that updating a record without touching this field does not update 005.
At the same time, a user could accidentally delete a character in the 005 when having focus. This mutilated field value will be saved however.
I would suggest to add a few lines to ModBiblioMarc in Biblio.pm to ascertain that adding or updating a record always updates 005 in marc21 as well as unimarc.
I will send a patch.
See: <a href="">
Patch sent: <a href="http://lists.koha-community.org/pipermail/koha-patches/2010-November/012854.html">click here</a>
Small adjustment: see http://lists.koha-community.org/pipermail/koha-patches/2010-November/012868.html
Created attachment 2861 [details] [review]
signed off patch
Pushed, please test