case tags: editing removes existing tags

when editing tags attached to cases, any existing tags are lost and only new ones are added. to recreate:

1. create a few case tags
2. create case with some of the tags
3. in the manage case screen, edit tags and add additional tags. the interface appears correct – the existing tags are reported as selected in the edit tags popup. but upon save, the tags are lost.

upon further testing, the behavior may be even more erratic (sometimes no tags are saved).

this was reproducible on the sandbox: http://sandbox.drupal.civicrm.org/civicrm/contact/view/case?action=view&reset=1&id=3&cid=137&context=case