Cannot access parent entity in hook_entity_create

+1 to this. I also have issues loading the temporary parent node entity in hook_field_widget_single_element_form_alter()

This work around for now is to use : $node = $parent_entity = $form_state->getFormObject()->getEntity();

Sample code below shows how to set the form default values as you enter paragraphs depending on how many.

/**
 * Implements hook_field_widget_single_element_PLUGIN_ID_form_alter().
 *
 * @see \Drupal\Core\Field\WidgetBase::formSingleElement()
 */
function my_module_field_widget_single_element_string_textfield_form_alter(array &$element, FormStateInterface $form_state, array $context) {
  $field_name = $context['items']->getName();
  $default_value = 'My default value to compare';
  if (($field_name == 'field_my_text_field')
    && ($context['items']->getEntity()->getEntityTypeId() == 'paragraph')
    && ($context['items']->getEntity()->bundle() == 'my_paragraph_type')) {
    $field_parents_no_subform = array_slice($element['#field_parents'], 0, -2);
    $parent_field_name = end($field_parents_no_subform);
    // Don't use $context['items']->getEntity()->getParentEntity()
    // it gets the current database node not the form temp node.
    // @see https://www.drupal.org/project/paragraphs/issues/3255456
    $parent_entity = $form_state->getFormObject()->getEntity();
    // Change new fields default to empty, by 
    if ($parent_entity->hasField($parent_field_name)) {
      $values = $parent_entity->get($parent_field_name)->getValue();
      if ((count($values) > 1) && ($element['value']['#default_value'] == $default_value)) {
        $element['value']['#default_value'] = '';
      }
    }
    // Change existing fields from default to empty.
    $input = $form_state->getUserInput();
    if (isset($input[$parent_field_name])) {
      $field_structure = array_merge(
        $element['#field_parents'],
        [
          $field_name,
          '0',
          'value',
        ]
      );
      // Get Value at
      // $input['field_parent'][$context['delta']]['subform'][$field_name]['0']['value'];.
      $current_value = NestedArray::getValue($input, $field_structure);
      if ($current_value == $default_value) {
        NestedArray::setValue($input, $field_structure, '');
        $form_state->setUserInput($input);
      }
    }
  }
}

Had the same "issue" when trying to alter the paragraphs field options in the allowed_values_function callback based on the parent entity. (node)

MR 46 worked great