Profile required field validation broken for

Steps to recreate on CiviCRM Drupal 7 demo site (http://drupal.demo.civicrm.org/):

1. Edit the user's "Name and Address" profile (http://drupal.demo.civicrm.org/user/4/edit/Name%20and%20Address)
2. Blank out one of the required fields, like "Last Name"
3. Enter data into one of the other fields, like "City (Home)"
4. Save the form

At this point, the expected result would be a required field validation warning, leaving the data entered into the form in place, so the user can correct the error and resubmit. Instead, the form submits and returns a "The changes have been saved." message. The required field isn't blanked out (which is correct), but the data entered in the non-required field is not saved (which is not correct).

Perhaps this is a permissions issue, but I haven't been able to figure out which permissions needs to be changed to get this to work. I suspect something else is going wrong.

Seems like a pretty critical bug since not only is the required field validation not working, which isn't a huge deal, but if the user has filled out the form and forgotten to fill out a required field, all the data they entered is not saved and they have to fill everything out again.