diff -u b/core/lib/Drupal/Core/Render/Element/MachineName.php b/core/lib/Drupal/Core/Render/Element/MachineName.php
--- b/core/lib/Drupal/Core/Render/Element/MachineName.php
+++ b/core/lib/Drupal/Core/Render/Element/MachineName.php
@@ -213,9 +213,6 @@
   /**
    * Form element validation handler for machine_name elements.
    *
-   * Note that #maxlength is validated by the form validator already, in
-   * \Drupal\Core\Form\FormValidatorInterface::validateForm().
-   *
    * This checks that the submitted value:
    * - Does not contain the replacement character only.
    * - Does not contain disallowed characters.
@@ -247,13 +244,10 @@
       }
     }
 
-    // Verify that the machine name is unique. Check form state if we need to
-    // explicitly validate the element. This happens every time when an error
-    // has been set, since this breaks on AJAX requests
-    if ($form_state->get('needs_revalidation') === $element['#name'] || $element['#default_value'] !== $element['#value']) {
+    // Verify that the machine name is unique.
+    if (isset($element['#value'])) {
       $function = $element['#machine_name']['exists'];
       if (call_user_func($function, $element['#value'], $element, $form_state)) {
-        $form_state->set('needs_revalidation', $element['#name']);
         $form_state->setError($element, t('The machine-readable name is already in use. It must be unique.'));
       }
     }
diff -u b/core/modules/media/tests/src/FunctionalJavascript/MediaTypeCreationTest.php b/core/modules/media/tests/src/FunctionalJavascript/MediaTypeCreationTest.php
--- b/core/modules/media/tests/src/FunctionalJavascript/MediaTypeCreationTest.php
+++ b/core/modules/media/tests/src/FunctionalJavascript/MediaTypeCreationTest.php
@@ -99,9 +99,6 @@
 
     // Create a new media type, which should create a new field we can reuse.
     $this->drupalGet('/admin/structure/media/add');
-    // Choose the source plugin _before_ setting the label and machine name in
-    // order to guard against the regression in
-    // https://www.drupal.org/project/drupal/issues/2557299.
     $page->selectFieldOption('Media source', 'test');
     $result = $assert_session->waitForElementVisible('css', 'fieldset[data-drupal-selector="edit-source-configuration"]');
     $this->assertNotEmpty($result);
@@ -113,10 +110,6 @@
     $mediaTypeMachineName = str_replace(' ', '_', strtolower($label));
 
     $this->drupalGet('admin/structure/media/add');
-
-    // Select the media source used by our media type. Do this before setting
-    // the label and machine name in order to guard against the regression in
-    // https://www.drupal.org/project/drupal/issues/2557299.
     $assert_session->fieldExists('Media source');
     $assert_session->optionExists('Media source', 'test');
     $page->selectFieldOption('Media source', 'test');
diff -u b/core/modules/system/tests/modules/form_test/src/Form/FormTestMachineNameValidationForm.php b/core/modules/system/tests/modules/form_test/src/Form/FormTestMachineNameValidationForm.php
--- b/core/modules/system/tests/modules/form_test/src/Form/FormTestMachineNameValidationForm.php
+++ b/core/modules/system/tests/modules/form_test/src/Form/FormTestMachineNameValidationForm.php
@@ -21,6 +21,10 @@
    * {@inheritdoc}
    */
   public function buildForm(array $form, FormStateInterface $form_state) {
+    // Disable client-side validation so that we can test AJAX requests with
+    // invalid input.
+    $form['#attributes']['novalidate'] = 'novalidate';
+
     $form['name'] = [
       '#type' => 'textfield',
       '#default_value' => $form_state->getValue('name'),
@@ -57,7 +61,6 @@
       ],
       '#required' => TRUE,
       '#ajax' => [
-        'trigger_as' => ['name' => 'snack_configure'],
         'callback' => '::buildAjaxSnackConfigureForm',
         'wrapper' => 'snack-config-form',
         'method' => 'replace',
@@ -71,30 +74,6 @@
       ],
       '#tree' => TRUE,
     ];
-    $form['snack_configure_button'] = [
-      '#type' => 'submit',
-      '#name' => 'snack_configure',
-      '#value' => 'Configure snack',
-      '#limit_validation_errors' => [['snack']],
-      '#validate' => ['::buildAjaxSnackConfigureFormValidate'],
-      '#submit' => ['::buildAjaxSnackConfigureFormSubmit'],
-      '#executes_submit_callback' => TRUE,
-      '#ajax' => [
-        'callback' => '::buildAjaxSnackConfigureForm',
-        'wrapper' => 'snack-config-form',
-      ],
-      '#attributes' => ['class' => ['js-hide']],
-    ];
-    $form['snack_configs']['apple']['#type'] = 'details';
-    $form['snack_configs']['apple']['#title'] = 'Configure Apple';
-    $form['snack_configs']['apple']['#open'] = TRUE;
-    $form['snack_configs']['pear']['#type'] = 'details';
-    $form['snack_configs']['pear']['#title'] = 'Configure pear';
-    $form['snack_configs']['pear']['#open'] = TRUE;
-    $form['snack_configs']['potato']['#type'] = 'details';
-    $form['snack_configs']['potato']['#title'] = 'Configure potato';
-    $form['snack_configs']['potato']['#open'] = TRUE;
-
     $form['submit'] = [
       '#type' => 'submit',
       '#value' => 'Save',
reverted:
--- b/core/modules/system/tests/src/Functional/Form/FormTest.php
+++ a/core/modules/system/tests/src/Functional/Form/FormTest.php
@@ -770,36 +770,4 @@
     $this->assertTrue(!empty($element), 'The textarea has the proper required attribute.');
   }
 
-  /**
-   * Tests that a machine name is still required after an AJAX submit.
-   *
-   * This protects against the regression in https://www.drupal.org/node/2557299.
-   */
-  public function testMachineNameRequiredFormAjaxSubmit() {
-    $this->drupalGet('/form-test/form-test-machine-name-validation');
-
-    $edit = [];
-    $this->submitForm($edit, 'Save');
-    $this->assertResponse(200);
-    $this->assertText('Machine-readable name field is required.');
-
-    $edit = [
-      'name' => 'test 1',
-      'id' => 'machine1',
-      'snack' => 'apple'
-    ];
-    $this->submitForm($edit, 'Save');
-    $this->assertResponse(200);
-    $this->assertText('The form_test_machine_name_validation_form form has been submitted successfully.');
-
-    $edit = [
-      'name' => 'test 2',
-      'id' => 'duplicate',
-      'snack' => 'potato',
-    ];
-    $this->submitForm($edit, 'snack_configure');
-    $this->submitForm($edit, 'Save');
-    $this->assertResponse(200);
-    $this->assertText('The machine-readable name is already in use. It must be unique.');
-  }
 }
only in patch2:
unchanged:
--- /dev/null
+++ b/core/modules/system/tests/src/FunctionalJavascript/MachineNameValidationTest.php
@@ -0,0 +1,46 @@
+<?php
+
+namespace Drupal\Tests\system\FunctionalJavascript;
+
+use Drupal\FunctionalJavascriptTests\JavascriptTestBase;
+
+/**
+ * Tests validation of machine name elements.
+ *
+ * @group Form
+ */
+class MachineNameValidationTest extends JavascriptTestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $modules = ['form_test'];
+
+  /**
+   * Tests that a machine name is still required after an AJAX submit.
+   *
+   * This guards against the regression in https://www.drupal.org/node/2557299.
+   */
+  public function testMachineNameRequiredFormAjaxSubmit() {
+    $assert = $this->assertSession();
+    $this->drupalGet('/form-test/form-test-machine-name-validation');
+
+    $assert->buttonExists('Save')->press();
+    $assert->pageTextContains('Machine-readable name field is required.');
+
+    $assert->fieldExists('Name')->setValue('test 1');
+    $assert->fieldExists('id')->setValue('test_1');
+    $assert->selectExists('snack')->selectOption('apple');
+    $assert->assertWaitOnAjaxRequest();
+    $assert->buttonExists('Save')->press();
+    $assert->pageTextContains('The form_test_machine_name_validation_form form has been submitted successfully.');
+
+    $assert->fieldExists('Name')->setValue('duplicate');
+    $this->assertJsCondition('document.forms[0].id.value === "duplicate"');
+    $assert->selectExists('snack')->selectOption('potato');
+    $assert->assertWaitOnAjaxRequest();
+    $assert->buttonExists('Save')->press();
+    $assert->pageTextContains('The machine-readable name is already in use. It must be unique.');
+  }
+
+}