When self-assessment is disabled for the workshop instance, it should say so on the allocation form: Add self-assessment - Self-assessment disabled. However, once the allocation is created, the text disappears from this static field. See screenshot. 

This is because both checkbox and static label share the same id: "addselfassessment". But, once allocation is created, this value is evaluated in instance_from_object function of random allocation lib.php:765. So it's explicitly set to 'false' thus losing the text value.

foreach (array('excludesamegroup', 'removecurrent', 'assesswosubmission', 'addselfassessment') as $k) {

  if (isset($data->$k)) {

    $i->$k = (bool)$data->$k;

  } else {

    $i->$k = false;

  }

}

A dirty fix would be to simply rename the static filed to something else in settings_form.php of random allocation, so it's different from the checkbox id.

if (empty($workshop->useselfassessment)) {

$mform->addElement('static', 'noselfassessment', get_string('addselfassessment', 'workshopallocation_random'),

get_string('selfassessmentdisabled', 'workshop'));

} else {

$mform->addElement('checkbox', 'addselfassessment', get_string('addselfassessment', 'workshopallocation_random'));

}

 
Steps to reproduce:

1. Create a workshop with 'Assessment settings: Use self-assessment: No'
2. Visit 'Allocate submissions' view from the workshop administration block
3. Switch to 'Scheduled allocation'
4. Ensure that Add self-assessments has a text value 'Self-assessment disabled'
5. Save the form and revisit the scheduled allocation
6. Ensure that the text is gone from the static field 'Add self-assessment'