Custom date format stopped working after updating from 7.x.2.0-alpha3

I have a custom date format that stopped working after updating the module. Actually, every custom date format I create is not working anymore when called with field_view_field().
The custom format is called 'blocco' and I'm calling field_view_field() like this:

field_view_field('node', $vars['node'], 'field_datetime', array('label' => 'hidden', 'type' => 'date_blocco'));

It seems that Drupal cannot find the proper formatter. I already cleared the caches, recreated the format and tried debugging with no luck.
Has anything changed in the rc version regarding this topic?