diff -u b/core/modules/field/tests/src/FunctionalJavascript/FormJSAddMoreTest.php b/core/modules/field/tests/src/FunctionalJavascript/FormJSAddMoreTest.php
--- b/core/modules/field/tests/src/FunctionalJavascript/FormJSAddMoreTest.php
+++ b/core/modules/field/tests/src/FunctionalJavascript/FormJSAddMoreTest.php
@@ -27,7 +27,7 @@
 
     $account = $this->drupalCreateUser([
       'view test entity',
-      'administer entity_test content'
+      'administer entity_test content',
     ]);
     $this->drupalLogin($account);
 
@@ -58,7 +58,7 @@
   /**
    * Tests the 'Add more' functionality.
    */
-  public function testFieldFormJSAddMore() {
+  public function testFieldFormJsAddMore() {
     $this->drupalGet('entity_test/add');
 
     $assert_session = $this->assertSession();
@@ -110,7 +110,8 @@
     // Validate the order of items conforms to the last drag action after a
     // updating the form via the server.
     $add_more_button->click();
-    $assert_session->assertWaitOnAjaxRequest();
+    $field_3 = $assert_session->waitForField('field_unlimited[3][value]');
+    $this->assertNotEmpty($field_3);
     $this->assertGreaterThan($field_weight_2->getValue(), $field_weight_0->getValue());
     $this->assertGreaterThan($field_weight_1->getValue(), $field_weight_2->getValue());