AJAX Wizard example cannot be extended with a managed_file

Problem/Motivation

Adding a managed_file field after step1 of the Wizard multi-step AJAX form example breaks the form. For the sake completeness and usefulness, this example should support all types of fields, including Managed Files.

Steps to reproduce

1. Enable the ajax_example module

2. Edit examples/modules/ajax_example/src/Form/Wizard.php by adding a new managed_file field on step2:

...
case 2:
        $limit_validation_errors = [['step'], ['step1']];
        $form['step1'] = [
          '#type' => 'value',
          '#value' => $form_state->getValue('step1'),
        ];
        $form['step2'] = [
          '#type' => 'fieldset',
          '#title' => $this->t('Step 2: Street address info'),
        ];
        $form['step2']['address'] = [
          '#type' => 'textfield',
          '#title' => $this->t('Your street address'),
          '#default_value' => $form_state->hasValue(['step2', 'address']) ? $form_state->getValue(['step2', 'address']) : '',
          '#required' => TRUE,
        ];
        $form['step2']['test_file'] = [
          '#type' => 'managed_file',
          '#upload_location' => 'public://attachments/',
          '#multiple' => TRUE,
          '#description' => 'This is a description',
          '#upload_validators' => [
            'file_validate_extensions' => [],
            'file_validate_size' => array(25600000)
          ],
          '#default_value' => $form_state->hasValue(["step2", 'test_file']) ? $form_state->getValue(["step2", 'test_file']) : [],
        ];
        break;
...

3. Navigate to your.site/examples/ajax-example/wizard
4. On the second step of the form, try to upload a File.
5. The form breaks (the ManagedFile field is gone)