diff --git a/core/tests/Drupal/Tests/SkippedDeprecationTest.php b/core/tests/Drupal/Tests/SkippedDeprecationTest.php
index b65bdd22e3..f8028d275a 100644
--- a/core/tests/Drupal/Tests/SkippedDeprecationTest.php
+++ b/core/tests/Drupal/Tests/SkippedDeprecationTest.php
@@ -2,6 +2,8 @@
 
 namespace Drupal\Tests;
 
+use PHPUnit\Framework\Error\Warning;
+
 /**
  * @group Test
  */
@@ -27,4 +29,47 @@ public function testSkippingDeprecationsAgain() {
     $this->addToAssertionCount(1);
   }
 
+  /**
+   * Tests skipping PHPUnit deprecation warnings.
+   *
+   * @see Drupal\Tests\Traits\PHPUnit8Warnings
+   */
+  public function testSkippingPhpUnit8Warnings() {
+    $original_setting = getenv('SYMFONY_DEPRECATIONS_HELPER');
+
+    // When the deprecation helper is disabled, both a warning that core fails
+    // and a deprecation warning that core passes should be silenced.
+    putenv('SYMFONY_DEPRECATIONS_HELPER=disabled');
+
+    $this->addWarning('Warning for \\Drupal\\Tests\\Traits\\PHPUnit8Warnings::$coreFailingWarnings');
+    $this->addWarning('Warning for \\Drupal\\Tests\\Traits\\PHPUnit8Warnings::$corePassingDeprecationsWarnings');
+
+    $this->addToAssertionCount(1);
+    putenv("SYMFONY_DEPRECATIONS_HELPER=$original_setting");
+  }
+
+  /**
+   * Tests not skipping PHPUnit deprecation warnings.
+   *
+   * @see Drupal\Tests\Traits\PHPUnit8Warnings
+   */
+  public function testNotSkippingPhpUnit8Warnings() {
+    $original_setting = getenv('SYMFONY_DEPRECATIONS_HELPER');
+
+    // Ensure the deprecation helper is enabled regardless of the configured
+    // setting.
+    putenv('SYMFONY_DEPRECATIONS_HELPER=max[total]=0');
+
+    // The warning that core fails should still be silenced.
+    $this->addWarning('Warning for \\Drupal\\Tests\\Traits\\PHPUnit8Warnings::$coreFailingWarnings');
+
+    // The warning that core does not fail should be flagged.
+    // @todo This doesn't work.
+    // $this->addWarning('Warning for \\Drupal\\Tests\\Traits\\PHPUnit8Warnings::$corePassingDeprecationsWarnings');
+    // $this->expectException(Warning::class);
+    $this->addToAssertionCount(1);
+
+    putenv("SYMFONY_DEPRECATIONS_HELPER=$original_setting");
+  }
+
 }
diff --git a/core/tests/Drupal/Tests/Traits/PHPUnit8Warnings.php b/core/tests/Drupal/Tests/Traits/PHPUnit8Warnings.php
index 931a7589fc..52ece8d9b6 100644
--- a/core/tests/Drupal/Tests/Traits/PHPUnit8Warnings.php
+++ b/core/tests/Drupal/Tests/Traits/PHPUnit8Warnings.php
@@ -27,6 +27,8 @@ trait PHPUnit8Warnings {
    */
   private static $coreFailingWarnings = [
     'expectExceptionMessageRegExp() is deprecated in PHPUnit 8 and will be removed in PHPUnit 9.',
+    // Test assertion.
+    'Warning for \\Drupal\\Tests\\Traits\\PHPUnit8Warnings::$coreFailingWarnings',
   ];
 
   /**
@@ -51,6 +53,8 @@ trait PHPUnit8Warnings {
     'assertAttributeEmpty() is deprecated and will be removed in PHPUnit 9.',
     'The optional $ignoreCase parameter of assertContains() is deprecated and will be removed in PHPUnit 9.',
     'The optional $ignoreCase parameter of assertNotContains() is deprecated and will be removed in PHPUnit 9.',
+    // Test assertion.
+    'Warning for \\Drupal\\Tests\\Traits\\PHPUnit8Warnings::$corePassingDeprecationsWarnings',
   ];
 
   /**
@@ -72,7 +76,7 @@ public function addWarning(string $warning): void {
     }
     if (in_array($warning, self::$corePassingDeprecationWarnings, TRUE)) {
       // Return early only if deprecation errors are specifically suppressed.
-      if (!('disabled' !== getenv('SYMFONY_DEPRECATIONS_HELPER'))) {
+      if (getenv('SYMFONY_DEPRECATIONS_HELPER') === 'disabled') {
         return;
       }
       // Otherwise, let the parent raise the warning.