When updating existing nodes, unmapped taxonomies have their values cleared

Situation:
I have a node type associated with 4 vocabularies, and having a number of cck fields. I have set up an importer that I wish to use to update certain node fields. The importer has a csv parser, and a node processor. The mapping includes the nid (set as unique target) some but not all the cck fields, and 3 of the 4 vocabularies.

Problem:
When performing an import, the mapped fields are all updated properly. The unmapped fields all conserve their values from before the import, except for the unmapped vocabulary. The node's existing term associations with the unmapped vocabulary seem to be cleared during import. The expected behavior is that the term associations would be conserved, as happens with values held in the unmapped cck fields.