Profile with only the email "erases" the display/sort name

I want to let existing contacts self register to a group. It doesn't need double opt-in, that's for an internal use.

I created a profile with only the email (they don't have login password), and checked the option to update the contact on duplicate:

http://drupal.demo.civicrm.org/civicrm/profile/create?gid=9&reset=1

If I type an existing email eg adamsanne@aol.org, it updates the right contact, BUT it erases the display name Adams anne and replaces it by "adamsanne@aol.org"

cf the forum http://forum.civicrm.org/index.php/topic,10860.0/topicseen.html for the discussion