Entity form does not pass changes to form state storage back to the parent form state

Under certain conditions, I see this error: Notice: Undefined index: items_count in Drupal\Core\Field\WidgetBase->formMultipleElements() (line 156 of core/lib/Drupal/Core/Field/WidgetBase.php

Steps to reproduce:

1. Make a content type B with an unlimited value text (or whatever) field.
2. Make a content type A with a single value required entityreference field that allows bundles of type B.
3. Set the form display for the entityreference field to use the Inline Entity Form - Simple widget.
4. On the content type A form, have some kind of AJAX element that will rebuild the form.
5. Create/edit content of type A, and use your AJAX functionality to refresh parts of the form.
6. Save the content.
(Notice should appear)

When WidgetBase is building the form in WidgetBase::form(), it first gets the widget state with WidgetBase::getWidgetState(). If it is empty (such as the first time it is called), it then populates the widget state with array_parents and items_count. That items_count is then relied on to be there, later in form processing. However, something about inline entity form's parent/child forms does not retain items_count.

if you are in EntityInlineForm::entityForm() after the child form is build, then items_count is set and exists in the child form state:
$child_form_state->getStorage()['field_storage']['#parents'][IEF_FIELD_NAME][0]['inline_entity_form']['#fields']['IEF_SUBFIELD_NAME']['items_count']

But when IEF is merging its child form and form state data back into the parent form, this data does not get transferred to its parent form.

By the end of form processing (e.g. in #after_build methods), items_count is always lost. This usually goes unnoticed, except if you have an ajax element on the page. Something about rebuilding the form ends up executing WidgetBase::formMultipleElements() on the parent form, which does not have the widget states of the fields within the inline form.

Full stacktrace:

Notice: Undefined index: items_count in Drupal\Core\Field\WidgetBase->formMultipleElements() (line 156 of core/lib/Drupal/Core/Field/WidgetBase.php).
Drupal\Core\Field\WidgetBase->formMultipleElements(Object, Array, Object)
Drupal\Core\Field\WidgetBase->form(Object, Array, Object)
Drupal\Core\Entity\Entity\EntityFormDisplay->buildForm(Object, Array, Object)
Drupal\Core\Entity\ContentEntityForm->form(Array, Object)
Drupal\pc_metadata\Form\MetadataForm->form(Array, Object)
Drupal\Core\Entity\EntityForm->buildForm(Array, Object)
Drupal\pc_metadata\Form\MetadataForm->buildForm(Array, Object)
Drupal\inline_entity_form\Form\EntityInlineForm->entityForm(Array, Object)
Drupal\inline_entity_form\Element\InlineEntityForm::processEntityForm(Array, Object, Array)
call_user_func_array(Array, Array)
Drupal\Core\Form\FormBuilder->doBuildForm('node_episode_form', Array, Object)
Drupal\Core\Form\FormBuilder->doBuildForm('node_episode_form', Array, Object)
Drupal\Core\Form\FormBuilder->doBuildForm('node_episode_form', Array, Object)
Drupal\Core\Form\FormBuilder->doBuildForm('node_episode_form', Array, Object)
Drupal\Core\Form\FormBuilder->doBuildForm('node_episode_form', Array, Object)
Drupal\Core\Form\FormBuilder->processForm('node_episode_form', Array, Object)
Drupal\Core\Form\FormBuilder->buildForm(Object, Object)
Drupal\Core\Entity\EntityFormBuilder->getForm(Object)
Drupal\node\Controller\NodeController->add(Object)
call_user_func_array(Array, Array)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}()
Drupal\Core\Render\Renderer->executeInRenderContext(Object, Object)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->wrapControllerExecutionInRenderContext(Array, Array)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}()
call_user_func_array(Object, Array)
Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object, 1)
Symfony\Component\HttpKernel\HttpKernel->handle(Object, 1, 1)
Drupal\Core\StackMiddleware\Session->handle(Object, 1, 1)
Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object, 1, 1)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object, 1, 1)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object, 1, 1)
Stack\StackedHttpKernel->handle(Object, 1, 1)
Drupal\Core\DrupalKernel->handle(Object)e(Object)