How to debug missing default value of managed file?
First of all can you try to remove the fieldset to a different $form[] array key? Is it really necessary to make it nested like this?

I think if you want to know if the file is added or removed then you can check in the following way.

$total_image_count = $form_state['storage']['image_elements']; // The number of image uploaded elements; just as example.
for($i = 0; $i &lt; $total_image_count; $i++) {
  if ($form_state['values']['image'][$i] != 0) {
    // File is not deleted.
  } elseif ($form_state['values']['image'][$i] == 0) {
    //File is deleted.
  }
}


However, check with this and let me know.