CSS is not loaded when Caching mode is enabled in Performance settings

Even though $type = 'module', $media = 'all', $preprocess = TRUE are set as defaults in drupal_add_css' parameters, omitting these options cause Drupal not to include module's CSS file.

temporary solution is updating
drupal_add_css(drupal_get_path('module', 'p6_helper') . '/p6_helper.css');
as
drupal_add_css(drupal_get_path('module', 'p6_helper') . '/p6_helper.css', 'module', 'all', TRUE);

Could be related, see: #235673: Changes to block caching mode not caught