Missing group audience field(og) from node add form by panel
It's a bug in "og/includes/og.field.inc". When the form is built twice "og_field_widget_form()" returns an empty array, thus the element has no options and seems to be missing. 

Here is a link to the bug tracker: https://www.drupal.org/node/2481709

Issue is still open. For now I just commented the offending if-block out.