text_format field has array('value' => null) as value

Problem/Motivation

In a multi-page webform we sometimes experience the problem that array('value' => null) is used as value. It seems to happen only when the field is not on the visible/submitted page, i.e. it's not part of $_POST. We've seen this problem at two spots:
1. On webform submission. This results in the string Array being persisted as value overriding a previous value, if any.
2. In \Drupal\editor\Editor. Here it results in a PHP error:

TypeError: strlen(): Argument #1 ($string) must be of type string, array given in strlen() (line 395 of /var/www/vhosts/civicrm.mintzukunft.de/httpdocs/pro/drupal/web/core/lib/Drupal/Component/Utility/Unicode.php).

At both spots I've added code to produce some debug logs. Interestingly at 2 the attribute #default_value in $element contains array('value' => null), though that's not what is defined in the webform. The field definition in the webform is like this:

field_name:
  '#type': text_format
  '#title': 'Some Title'
  '#default_value':
    value: '<p>Some Text</p>'
    format: filtered_html
  '#allowed_formats':
    filtered_html: filtered_html

By contrast $element contains (JSON encoded and reduced to only some properties):

{
    "#type": "text_format",
    "#default_value": {
        "value": null
    },
    "#allowed_formats": {
        "filtered_html": "filtered_html"
    },
    "#webform_plugin_id": "text_format",
    "value": {
        "#type": "textarea",
        "#default_value": {
            "value": null
        },
        "#allowed_formats": {
            "filtered_html": "filtered_html"
        },
        "#webform_plugin_id": "text_format",
        "#value": {
            "value": null
        },
    },
    "format": {
        "#type": "container",
        "format": {
            "#type": "select",
            "#default_value": "filtered_html",
            "#value": "filtered_html",
        },
        "editor": {
            "#type": "hidden",
            "#value": "filtered_html",
        }
    },
    "#format": "filtered_html",
}

Steps to reproduce

Unfortunately I wasn't able to reproduce it myself, yet.

Currently the only thing I was able to workaround the issue was adding a `preSave()` method to \Drupal\webform\Plugin\WebformElement\TextFormat that replaces the value with the previously stored value or the empty string, and to modify \Drupal\editor\Element::preRenderTextFormat.