The course certificate does not uses the course completion api to calculate if the course was completed or not.

$result = $DB->get_field('course_completions', 'timecompleted', ['course' => $course->id, 'userid' => $user->id]);

$completiondate = $result ? userdate($result, get_string('strftimedatefullshort')) : "";

In some cases, the course_completion record exist, with empty completion dates, so the student gets a certificate (with empty completion date and empty grades) while he/she should not.

Perhaps there should be an extra check to see if the course was completed before issuing a certificate?

$completioninfo = new \completion_info($course);

if ($completioninfo->is_enabled() && $completioninfo->is_course_complete($user->id)) {

   // issue certificate

}