Problem with field visibility after upgrade using Content Migrate

I'm the maintainer of the Metis module and I just released a D7 version of the module. It uses a CCK field (D6) or a core field (D7) to save some data in a text field. For that reason I use the CCK's content migrate module to migrate the content of the field. It seems to work fine, but there is a strange thing happening regarding the field's visibility:

After the upgrade the field isn't rendered until I visit the content type's display settings page (admin/structure/types/manage/[content_type]/display/default), move the field, and save.

I investigated and found out that doing so, something is changed in the field's visibility settings. These are parts of the results of var_export(field_info_instance('node', 'field_metis', '[content_type]')); before and after saving the display settings page:

BEFORE:

    'default' =>
    array (
      'label' => 'hidden',
      'type' => 'taxonomy_term_reference_plain',
      'weight' => '22',
      'settings' =>
      array (
      ),
      'module' => 'taxonomy',
    ),

AFTER:

    'default' =>
    array (
      'label' => 'hidden',
      'type' => 'metis_default',
      'weight' => '13',
      'settings' =>
      array (
      ),
      'module' => 'metis',
    ),

Now I'm wondering if that taxonomy_term_reference_plain is due to an error in the Content Migrate module or if I need to change something in the Metis module to make the upgrade work smoothly. Can you help?