final question doesn't respect #submit

I created a custom module to record some statistics after each answer is submitted and discovered my custom submit handler doesn't fire after the final question.

The following code works as expected for all questions except for the last:

function custom_form_alter(&$form, &$form_state, $form_id) {
  switch($form_id) {
    case 'quiz_question_answering_form':
      $form['#submit'][] = 'custom_submit_handler';
      break;
  }
}

Prepending the #submit array doesn't make a difference:

array_unshift($form['#submit'], 'custom_submit_handler');

I've never experienced this when using #submit previously and suspect a conditional "if last question" redirect is occurring somewhere in the Quiz module. I haven't look at the code yet, but I wanted to log the bug and raise awareness.