Validation problem with new submission API

I'm updating Webform REST to use the new submission API introduced here: #2871886: Provide an API to programmatically validate and submit a webform submission

I've run into a few problems.

Example Code

The example code suggests the following:

// Check that the webform is open.
$webform = \Drupal\webform\entity\Webform::load('initial_screening'); 
$is_open = \Drupal\webform\WebformSubmissionForm::isOpen($values);

I believe this should be:

// Check that the webform is open.
$webform = \Drupal\webform\entity\Webform::load('initial_screening'); 
$is_open = \Drupal\webform\WebformSubmissionForm::isOpen($webform);

Validation

Without looking into it in any great detail, validation seems too lenient. For example, I have a number of taxonomy reference fields which seem to validate OK even if the term ID is not within the specified vocabulary.

Current working code which picks up invalid taxonomy IDs:

    // Create webform submission object.
    $webform_submission = WebformSubmission::create(['webform_id' => $webform_data['webform_id']]);

    // Don't submit webform ID.
    unset($webform_data['webform_id']);

    // Get the form object.
    $entity_form_object = \Drupal::entityTypeManager()
      ->getFormObject('webform_submission', 'default');
    $entity_form_object->setEntity($webform_submission);

    // Initialize the form state.
    $form_state = (new FormState())->setValues($webform_data);

    // Submit form.
    \Drupal::formBuilder()->submitForm($entity_form_object, $form_state);

    $errors = $form_state->getErrors();

    // Check there are no validation errors.
    if (!empty($errors)) {
      $errors = ['error' => $errors];
      return new ResourceResponse($errors);
    }

New code which validates even with invalid selections:

    if ($is_open === TRUE) {
      // Validate submission.
      $errors = \Drupal\webform\WebformSubmissionForm::validateValues($values);

      // Check there are no validation errors.
      if (!empty($errors)) {
        $errors = ['error' => $errors];
        return new ResourceResponse($errors);
      }
      else {
        $webform_submission = \Drupal\webform\WebformSubmissionForm::submitValues($values);
        return new ResourceResponse(['sid' => $webform_submission->id()]);
      }

    }

Viewing Submissions

After a successful submission using invalid term IDs, I can no longer view submissions for that webform. The error is presumably related to trying to get the label of a non existent taxonomy term:

 PHP Fatal error:  Call to a member function label() on a non-object in /modules/webform/src/Plugin/WebformElement/WebformEntityReferenceTrait.php on line 68, referer: http://local/admin/structure/webform

Submission data

$values array:

Array
(
    [webform_id] => initial_screening
    [entity_type] => 
    [entity_id] => 
    [in_draft] => 
    [uri] => /webform/initial_screening/api
    [data] => Array
        (
            [user_session] => 9
            [substances] => Array
                (
                    [0] => 35
                )

            [age] => 134
            [gender] => 13
        )

)