Form error on layout section when using module layout_section_classes

We are also seeing this when we extend the core layout_builder module's MultiWidthLayoutBase class.

When we switch layouts in the layout section pop-up and there the option selected for the current layout doesn't exist in the getWidthOptions() for the new layout, it spits out this error. It doesn't appear to have any affect and the error is actually coming from the submitConfigurationForm() method, and not buildConfigurationForm().

For example, we have a three column and two column layouts both extending the MultiWidthLayoutBase:

Three column:

class ThreeColumn extends MultiWidthLayoutBase implements PluginFormInterface {
  protected function getWidthOptions() {
    return [
      '100' => '100%'
    ];
  }
}

Two column:

class TwoColumn extends MultiWidthLayoutBase implements PluginFormInterface {
  protected function getWidthOptions() {
    return [
      '50-50' => '50% - 50%',
      '33-67' => '33% - 67%',
      '67-33' => '67% - 33%',
    ];
  }
}

When we switch from ThreeColumn to TwoColumn it complains because the 100 option doesn't exist in the TwoColumn (and vice-versa).

To expand on this, I noticed in the LayoutParagraphsBehavior buildBehaviorForm() method there is a supposed to be an ajax callback that fires (ajaxUpdateOptions) when a layout is selected:

    $form['layout'] = [
      '#title' => $this->t('Choose a layout:'),
      '#type' => 'layout_select',
      '#options' => $available_layouts,
      '#default_value' => $default_value,
      '#ajax' => [
        'wrapper' => $wrapper_id,
        'callback' => [$this, 'ajaxUpdateOptions'],
        'progress' => [
          'type' => 'throbber',
        ],
      ],
      '#weight' => 0,
    ];

The callback itself seems like it's supposed to rebuild the $form['config'] options (where the width options are):

  public function ajaxUpdateOptions(array $form, FormStateInterface $form_state) {
    $triggering_element = $form_state->getTriggeringElement();
    $parents = $triggering_element['#parents'];
    array_splice($parents, -1, 1, ['config']);
    $config_form = NestedArray::getValue($form, $parents);
    if (isset($config_form)) {
      return $config_form;
    }
    return [];
  }

However, when a layout is selected this callback is not firing, and the submitBehaviorForm() method is ran instead. I'm not sure if that's how it's supposed to work or not.

It looks like this error isn't "harmless" but actually causes paragraphs to be lost when switching between different layouts that have existing paragraphs in regions. Linked relevant issue.