Upgrading Textarea to Formatted Text and config import does not clear past submissions

Looking into this yes It seems when its rendering the submissions as soon as it gets past the new submissions which had 2 text areas with a conditional field. I get this error when its rendering out a submission from the old form that only had 1 text area and not conditionally hidden fields.

Locally I could get this fixed by adding

    // if value is array convert to string
    if (is_array($value)) {
      $value = implode(", ", $value);
    }

to

  /**
   * {@inheritdoc}
   */
  protected function formatHtmlItem(array $element, WebformSubmissionInterface $webform_submission, array $options = []) {
    $value = $this->getValue($element, $webform_submission, $options);

    $format = $value['format'] ?? $this->getItemFormat($element);
    $value = $value['value'] ?? $value;
    // if value is array convert to string
    if (is_array($value)) {
      $value = implode(", ", $value);
    }
    switch ($format) {
      case 'raw':
        return $value;

      case 'value':
        $default_format = filter_default_format(User::load($webform_submission->getOwnerId()));
        return check_markup($value, $default_format);

      default:
        return check_markup($value, $format);
    }
  }

Can you post a very simple example webform which can be used to replicate this issue?

Ok so diving into this a bit more over the last hour I can't reproduce the issue in a new form, but I think I have a better Idea as to what is going on.

What happened is a normal text area was "upgraded" to a WYSWIG, and it appears the same machine name was kept.

When I do this locally, and I mean
1. Delete the old text area
2. Readd the same field but as a WYSWIG field

I don't get an error.

But I think when we exported this change to config, and migrated it to prod is it possible that the migration process did not actually clear out the old plain text submissions?

Yeah so steps to reproduce.

1. Start with a simple text are a form and submit a test submission

elements: |-
  title:
    '#type': textfield
    '#title': title
  site_contact_details:
    '#type': textarea
    '#title': 'site contact details'

2. Then import (drush cim) a new version of that webform that makes the field become a WYSWIG text area.

elements: |-
  title:
    '#type': textfield
    '#title': title
  site_contact_details:
    '#type': text_format
    '#title': 'site contact details'
    '#allowed_formats':
      webform_html: webform_html

3. Do another test submission

4. See errror on submisison page

So yeah I guess this was my mistake for keeping the same machine name, but also seems like a fairly common foot gun that could happen to others.

This feels like an edge case related to changing the elements via YAML. For now, I would like to not fix it.