diff -u b/core/modules/system/src/Tests/Entity/EntityFormCorrectUserInputMappingOnFieldDeltaElementsTest.php b/core/modules/system/src/Tests/Entity/EntityFormCorrectUserInputMappingOnFieldDeltaElementsTest.php
--- b/core/modules/system/src/Tests/Entity/EntityFormCorrectUserInputMappingOnFieldDeltaElementsTest.php
+++ b/core/modules/system/src/Tests/Entity/EntityFormCorrectUserInputMappingOnFieldDeltaElementsTest.php
@@ -97,8 +97,13 @@
       'field_reference[0][_weight]' => 0,
       'field_reference[1][_weight]' => -1,
     );
+    // Executing an ajax call is important before saving as it will trigger
+    // form state caching and so if for any reasons the form is rebuilt with
+    // the entity built based on the user submitted values then the correct
+    // 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->drupalPostForm(NULL, $edit, t('Save'));
+    $this->drupalPostForm(NULL, [], t('Save'));
 
     $entity1 = $storage->load($entity1->id());
 
diff -u b/core/modules/system/tests/modules/entity_test/src/Plugin/Field/FieldType/EntityReferenceWithNameTestItem.php b/core/modules/system/tests/modules/entity_test/src/Plugin/Field/FieldType/EntityReferenceWithNameTestItem.php
--- b/core/modules/system/tests/modules/entity_test/src/Plugin/Field/FieldType/EntityReferenceWithNameTestItem.php
+++ b/core/modules/system/tests/modules/entity_test/src/Plugin/Field/FieldType/EntityReferenceWithNameTestItem.php
@@ -40,13 +40,6 @@
   /**
    * {@inheritdoc}
    */
-  public static function mainPropertyName() {
-    return NULL;
-  }
-
-  /**
-   * {@inheritdoc}
-   */
   public static function schema(FieldStorageDefinitionInterface $field_definition) {
     $schema = parent::schema($field_definition);
     $schema['columns']['name'] = [