Why would pieces of content of each content type be migrated but their text formats wrong?

I'm trying to migrate content D6 -> D7. I've written the importers (actually just edited existing ones) and run the process. However, as far as I can see, body of content are imported but with the wrong format i.e. one of them correctly shows as Full HTML while another shows just Select, as per the below screenshot.

Below is my importer:

class StoryMigration extends NodeMigration {
  public function __construct(array $arguments) {
    parent::__construct($arguments);
    $this->addFieldMapping('title', 'title');
    $this->addFieldMapping('body', 'body');
    $this->addFieldMapping('taxonomy_tags_1', 'field_tags')
      ->defaultValue('tid');
    $this->addFieldMapping('field_headerimage', 'field_headerimage')
      ->sourceMigration('Files');
    $this->addFieldMapping('field_headerimage:file_class')
      ->defaultValue('MigrateFileFid');
  }
}

UPDATE: I also did a format mapping, like below:

$common_arguments = array(
    'source_connection' => 'default',
    'source_version' => 6,
    'format_mappings' => array(
      '1' => 'filtered_html',
      '2' => 'full_html',
      '3' => 'php_code'
    ),
   ...
   ...
   ...
  );

How can I pull this off?

Best,
K.