Contact import ‘fill’ mode does not seem to fill empty fields correctly.

One of my clients recently moved to CiviCRM from a third-party glorified elist.  At first they could not even recover the street address or city field from their elist server, so I set up CIviCRM, imported what data they had, then sent an elist mailing to allow people to update their contact information online.

A few weeks later my client managed to recover the street and city data (after divine/admin intervention).  I attempted to import street address and city fields using the ‘fill’ mode so as not to overwrite fields in records that had already been personally updated online.  CiviCRM did not fill in any new information, even though the matching criteria worked perfectly - all records were tagged with a temporary tag I created for the import.

I was able to work around this issue by manually editing out lines in the import file for the 100+ contacts that had already self-updated, then re-importing the file using the ‘update’ mode. 

I've only tested this on recent versions of CiviCRM for WordPress.