diff -u b/core/modules/system/tests/modules/entity_test/src/Plugin/Field/FieldWidget/ShapeOnlyColorEditableWidget.php b/core/modules/system/tests/modules/entity_test/src/Plugin/Field/FieldWidget/ShapeOnlyColorEditableWidget.php
--- b/core/modules/system/tests/modules/entity_test/src/Plugin/Field/FieldWidget/ShapeOnlyColorEditableWidget.php
+++ b/core/modules/system/tests/modules/entity_test/src/Plugin/Field/FieldWidget/ShapeOnlyColorEditableWidget.php
@@ -3,6 +3,7 @@
 namespace Drupal\entity_test\Plugin\Field\FieldWidget;
 
 use Drupal\Core\Field\FieldItemListInterface;
+use Drupal\Core\Field\FieldStorageDefinitionInterface;
 use Drupal\Core\Field\WidgetBase;
 use Drupal\Core\Form\FormStateInterface;
 
@@ -39,2 +40,29 @@
 
+  protected function formMultipleElements(FieldItemListInterface $items, array &$form, FormStateInterface $form_state){
+    $cardinality = $this->fieldDefinition->getFieldStorageDefinition()->getCardinality();
+
+    // Determine the number of widgets to display.
+    switch ($cardinality) {
+      case FieldStorageDefinitionInterface::CARDINALITY_UNLIMITED:
+        $field_name = $this->fieldDefinition->getName();
+        $parents = $form['#parents'];
+
+        $field_state = static::getWidgetState($parents, $field_name, $form_state);
+        $max = $field_state['items_count'];
+        break;
+
+      default:
+        $max = $cardinality - 1;
+        break;
+    }
+
+    for ($delta = 0; $delta <= $max; $delta++) {
+      // Add a new empty item if it doesn't exist yet at this delta.
+      if (!isset($items[$delta])) {
+        $items->appendItem(['shape' => 'circle', 'color' => 'red']);
+      }
+    }
+
+    return parent::formMultipleElements($items, $form, $form_state);
+  }
 }
diff -u b/core/tests/Drupal/FunctionalTests/Entity/EntityFormCorrectUserInputMappingOnFieldDeltaElementsTest.php b/core/tests/Drupal/FunctionalTests/Entity/EntityFormCorrectUserInputMappingOnFieldDeltaElementsTest.php
--- b/core/tests/Drupal/FunctionalTests/Entity/EntityFormCorrectUserInputMappingOnFieldDeltaElementsTest.php
+++ b/core/tests/Drupal/FunctionalTests/Entity/EntityFormCorrectUserInputMappingOnFieldDeltaElementsTest.php
@@ -96,6 +96,10 @@
     // mapping will break after the form builder maps over the new form the
     // user submitted values based on the previous delta ordering.
     $this->drupalPostForm(NULL, $edit, t('Add another item'));
+    $this->assertRaw('name="shape[2][shape]" value="circle"');
+    $this->assertRaw('name="shape[2][color]" value="red"');
+    $this->assertRaw('name="shape[3][shape]" value="circle"');
+    $this->assertRaw('name="shape[3][color]" value="red"');
     $this->drupalPostForm(NULL, [], t('Save'));
 
     // Reload the entity.
@@ -106,6 +110,8 @@
     $this->assertEquals($entity->get($this->fieldName)->getValue(), [
       ['shape' => 'circle', 'color' => 'blue'],
       ['shape' => 'rectangle', 'color' => 'green'],
+      ['shape' => 'circle', 'color' => 'red'],
+      ['shape' => 'circle', 'color' => 'red'],
     ]);
   }