$mform->addElement('course', 'id_course', 'Course');
$mform->addRule('id_course', 'Required', 'required');
$mform->hideIf('id_course', 'template_code', 'neq', 'course');
 
This bug leads to the fact that form elements are hidden and the user does not see them - however, the form does not pass validation and errors are not displayed - as a result, the user does not understand what is happening

Solution: hideIf - should disable execution of validation rules on elements hidden by it