Taxonomy term description is emptied on save

I am trying to save a taxonomy term, and the description field (which is a "Long text and summary") doesn't get saved.
I have managed to track the issue to this function, which is found in title.core.inc

/**
 * Sync back callback for the text with summary field type.
 */
function title_field_text_with_summary_sync_set($entity_type, $entity, $legacy_field, $info, $langcode) {
  $format_key = $info['additional keys']['format'];kpr($info['field']['field_name']);kpr($entity);kpr($legacy_field);kpr($format_key);
  $entity->{$info['field']['field_name']}[$langcode][0]['value'] = $entity->{$legacy_field};
  $entity->{$info['field']['field_name']}[$langcode][0]['format'] = $entity->{$format_key};
}

For whatever reason the description_field is replaced with description, which as you can see from the attached screenshot (desc-debug.png), it's empty.