diff --git a/core/lib/Drupal/Component/Utility/UrlHelper.php b/core/lib/Drupal/Component/Utility/UrlHelper.php
index 34a4dbe62e4..3127eaba641 100644
--- a/core/lib/Drupal/Component/Utility/UrlHelper.php
+++ b/core/lib/Drupal/Component/Utility/UrlHelper.php
@@ -46,7 +46,7 @@ class UrlHelper {
     $params = [];
 
     foreach ($query as $key => $value) {
-      $key = ($parent ? $parent . '[' . rawurlencode($key) . ']' : rawurlencode($key));
+      $key = ($parent ? $parent . rawurlencode('[' . $key . ']') : rawurlencode($key));
 
       // Recurse into children.
       if (is_array($value)) {
diff --git a/core/modules/big_pipe/src/Tests/BigPipePlaceholderTestCases.php b/core/modules/big_pipe/src/Tests/BigPipePlaceholderTestCases.php
index 69ec86fdc25..46377800bca 100644
--- a/core/modules/big_pipe/src/Tests/BigPipePlaceholderTestCases.php
+++ b/core/modules/big_pipe/src/Tests/BigPipePlaceholderTestCases.php
@@ -59,29 +59,29 @@ class BigPipePlaceholderTestCases {
         ],
       ]
     );
-    $status_messages->bigPipePlaceholderId = 'callback=Drupal%5CCore%5CRender%5CElement%5CStatusMessages%3A%3ArenderMessages&args[0]&token=_HAdUpwWmet0TOTe2PSiJuMntExoshbm1kh2wQzzzAA';
+    $status_messages->bigPipePlaceholderId = 'callback=Drupal%5CCore%5CRender%5CElement%5CStatusMessages%3A%3ArenderMessages&args%5B0%5D&token=_HAdUpwWmet0TOTe2PSiJuMntExoshbm1kh2wQzzzAA';
     $status_messages->bigPipePlaceholderRenderArray = [
-      '#markup' => '<span data-big-pipe-placeholder-id="callback=Drupal%5CCore%5CRender%5CElement%5CStatusMessages%3A%3ArenderMessages&amp;args[0]&amp;token=_HAdUpwWmet0TOTe2PSiJuMntExoshbm1kh2wQzzzAA"></span>',
+      '#markup' => '<span data-big-pipe-placeholder-id="callback=Drupal%5CCore%5CRender%5CElement%5CStatusMessages%3A%3ArenderMessages&amp;args%5B0%5D&amp;token=_HAdUpwWmet0TOTe2PSiJuMntExoshbm1kh2wQzzzAA"></span>',
       '#cache' => $cacheability_depends_on_session_and_nojs_cookie,
       '#attached' => [
         'library' => ['big_pipe/big_pipe'],
         'drupalSettings' => [
           'bigPipePlaceholderIds' => [
-            'callback=Drupal%5CCore%5CRender%5CElement%5CStatusMessages%3A%3ArenderMessages&args[0]&token=_HAdUpwWmet0TOTe2PSiJuMntExoshbm1kh2wQzzzAA' => TRUE,
+            'callback=Drupal%5CCore%5CRender%5CElement%5CStatusMessages%3A%3ArenderMessages&args%5B0%5D&token=_HAdUpwWmet0TOTe2PSiJuMntExoshbm1kh2wQzzzAA' => TRUE,
           ],
         ],
         'big_pipe_placeholders' => [
-          'callback=Drupal%5CCore%5CRender%5CElement%5CStatusMessages%3A%3ArenderMessages&amp;args[0]&amp;token=_HAdUpwWmet0TOTe2PSiJuMntExoshbm1kh2wQzzzAA' => $status_messages->placeholderRenderArray,
+          'callback=Drupal%5CCore%5CRender%5CElement%5CStatusMessages%3A%3ArenderMessages&amp;args%5B0%5D&amp;token=_HAdUpwWmet0TOTe2PSiJuMntExoshbm1kh2wQzzzAA' => $status_messages->placeholderRenderArray,
         ],
       ],
     ];
-    $status_messages->bigPipeNoJsPlaceholder = '<span data-big-pipe-nojs-placeholder-id="callback=Drupal%5CCore%5CRender%5CElement%5CStatusMessages%3A%3ArenderMessages&amp;args[0]&amp;token=_HAdUpwWmet0TOTe2PSiJuMntExoshbm1kh2wQzzzAA"></span>';
+    $status_messages->bigPipeNoJsPlaceholder = '<span data-big-pipe-nojs-placeholder-id="callback=Drupal%5CCore%5CRender%5CElement%5CStatusMessages%3A%3ArenderMessages&amp;args%5B0%5D&amp;token=_HAdUpwWmet0TOTe2PSiJuMntExoshbm1kh2wQzzzAA"></span>';
     $status_messages->bigPipeNoJsPlaceholderRenderArray = [
-      '#markup' => '<span data-big-pipe-nojs-placeholder-id="callback=Drupal%5CCore%5CRender%5CElement%5CStatusMessages%3A%3ArenderMessages&amp;args[0]&amp;token=_HAdUpwWmet0TOTe2PSiJuMntExoshbm1kh2wQzzzAA"></span>',
+      '#markup' => '<span data-big-pipe-nojs-placeholder-id="callback=Drupal%5CCore%5CRender%5CElement%5CStatusMessages%3A%3ArenderMessages&amp;args%5B0%5D&amp;token=_HAdUpwWmet0TOTe2PSiJuMntExoshbm1kh2wQzzzAA"></span>',
       '#cache' => $cacheability_depends_on_session_and_nojs_cookie,
       '#attached' => [
         'big_pipe_nojs_placeholders' => [
-          '<span data-big-pipe-nojs-placeholder-id="callback=Drupal%5CCore%5CRender%5CElement%5CStatusMessages%3A%3ArenderMessages&amp;args[0]&amp;token=_HAdUpwWmet0TOTe2PSiJuMntExoshbm1kh2wQzzzAA"></span>' => $status_messages->placeholderRenderArray,
+          '<span data-big-pipe-nojs-placeholder-id="callback=Drupal%5CCore%5CRender%5CElement%5CStatusMessages%3A%3ArenderMessages&amp;args%5B0%5D&amp;token=_HAdUpwWmet0TOTe2PSiJuMntExoshbm1kh2wQzzzAA"></span>' => $status_messages->placeholderRenderArray,
         ],
       ],
     ];
@@ -109,7 +109,7 @@ class BigPipePlaceholderTestCases {
         [
           'command' => 'insert',
           'method' => 'replaceWith',
-          'selector' => '[data-big-pipe-placeholder-id="callback=Drupal%5CCore%5CRender%5CElement%5CStatusMessages%3A%3ArenderMessages&args[0]&token=_HAdUpwWmet0TOTe2PSiJuMntExoshbm1kh2wQzzzAA"]',
+          'selector' => '[data-big-pipe-placeholder-id="callback=Drupal%5CCore%5CRender%5CElement%5CStatusMessages%3A%3ArenderMessages&args%5B0%5D&token=_HAdUpwWmet0TOTe2PSiJuMntExoshbm1kh2wQzzzAA"]',
           'data' => "\n" . '    <div role="contentinfo" aria-label="Status message" class="messages messages--status">' . "\n" . '                  <h2 class="visually-hidden">Status message</h2>' . "\n" . '                    Hello from BigPipe!' . "\n" . '            </div>' . "\n    ",
           'settings' => NULL,
         ],
@@ -272,24 +272,24 @@ class BigPipePlaceholderTestCases {
         '#lazy_builder' => ['\Drupal\big_pipe_test\BigPipeTestController::exception', ['llamas', 'suck']],
       ]
     );
-    $exception->bigPipePlaceholderId = 'callback=%5CDrupal%5Cbig_pipe_test%5CBigPipeTestController%3A%3Aexception&amp;args[0]=llamas&amp;args[1]=suck&amp;token=uhKFNfT4eF449_W-kDQX8E5z4yHyt0-nSHUlwaGAQeU';
+    $exception->bigPipePlaceholderId = 'callback=%5CDrupal%5Cbig_pipe_test%5CBigPipeTestController%3A%3Aexception&amp;args%5B0%5D=llamas&amp;args%5B1%5D=suck&amp;token=uhKFNfT4eF449_W-kDQX8E5z4yHyt0-nSHUlwaGAQeU';
     $exception->bigPipePlaceholderRenderArray = [
-      '#markup' => '<span data-big-pipe-placeholder-id="callback=%5CDrupal%5Cbig_pipe_test%5CBigPipeTestController%3A%3Aexception&amp;args[0]=llamas&amp;args[1]=suck&amp;token=uhKFNfT4eF449_W-kDQX8E5z4yHyt0-nSHUlwaGAQeU"></span>',
+      '#markup' => '<span data-big-pipe-placeholder-id="callback=%5CDrupal%5Cbig_pipe_test%5CBigPipeTestController%3A%3Aexception&amp;args%5B0%5D=llamas&amp;args%5B1%5D=suck&amp;token=uhKFNfT4eF449_W-kDQX8E5z4yHyt0-nSHUlwaGAQeU"></span>',
       '#cache' => $cacheability_depends_on_session_and_nojs_cookie,
       '#attached' => [
         'library' => ['big_pipe/big_pipe'],
         'drupalSettings' => [
           'bigPipePlaceholderIds' => [
-            'callback=%5CDrupal%5Cbig_pipe_test%5CBigPipeTestController%3A%3Aexception&args[0]=llamas&args[1]=suck&token=uhKFNfT4eF449_W-kDQX8E5z4yHyt0-nSHUlwaGAQeU' => TRUE,
+            'callback=%5CDrupal%5Cbig_pipe_test%5CBigPipeTestController%3A%3Aexception&args%5B0%5D=llamas&args%5B1%5D=suck&token=uhKFNfT4eF449_W-kDQX8E5z4yHyt0-nSHUlwaGAQeU' => TRUE,
           ],
         ],
         'big_pipe_placeholders' => [
-          'callback=%5CDrupal%5Cbig_pipe_test%5CBigPipeTestController%3A%3Aexception&amp;args[0]=llamas&amp;args[1]=suck&amp;token=uhKFNfT4eF449_W-kDQX8E5z4yHyt0-nSHUlwaGAQeU' => $exception->placeholderRenderArray,
+          'callback=%5CDrupal%5Cbig_pipe_test%5CBigPipeTestController%3A%3Aexception&amp;args%5B0%5D=llamas&amp;args%5B1%5D=suck&amp;token=uhKFNfT4eF449_W-kDQX8E5z4yHyt0-nSHUlwaGAQeU' => $exception->placeholderRenderArray,
         ],
       ],
     ];
     $exception->embeddedAjaxResponseCommands = NULL;
-    $exception->bigPipeNoJsPlaceholder = '<span data-big-pipe-nojs-placeholder-id="callback=%5CDrupal%5Cbig_pipe_test%5CBigPipeTestController%3A%3Aexception&amp;args[0]=llamas&amp;args[1]=suck&amp;token=uhKFNfT4eF449_W-kDQX8E5z4yHyt0-nSHUlwaGAQeU"></span>';
+    $exception->bigPipeNoJsPlaceholder = '<span data-big-pipe-nojs-placeholder-id="callback=%5CDrupal%5Cbig_pipe_test%5CBigPipeTestController%3A%3Aexception&amp;args%5B0%5D=llamas&amp;args%5B1%5D=suck&amp;token=uhKFNfT4eF449_W-kDQX8E5z4yHyt0-nSHUlwaGAQeU"></span>';
     $exception->bigPipeNoJsPlaceholderRenderArray = [
       '#markup' => $exception->bigPipeNoJsPlaceholder,
       '#cache' => $cacheability_depends_on_session_and_nojs_cookie,
diff --git a/core/modules/big_pipe/src/Tests/BigPipeTest.php b/core/modules/big_pipe/src/Tests/BigPipeTest.php
index 9d2b25734e3..a4d5562008d 100644
--- a/core/modules/big_pipe/src/Tests/BigPipeTest.php
+++ b/core/modules/big_pipe/src/Tests/BigPipeTest.php
@@ -293,7 +293,7 @@ class BigPipeTest extends WebTestBase {
     // @see performMetaRefresh()
 
     $this->drupalGet(Url::fromRoute('big_pipe_test_multi_occurrence'));
-    $big_pipe_placeholder_id = 'callback=Drupal%5CCore%5CRender%5CElement%5CStatusMessages%3A%3ArenderMessages&amp;args[0]&amp;token=_HAdUpwWmet0TOTe2PSiJuMntExoshbm1kh2wQzzzAA';
+    $big_pipe_placeholder_id = 'callback=Drupal%5CCore%5CRender%5CElement%5CStatusMessages%3A%3ArenderMessages&amp;args%5B0%5D&amp;token=_HAdUpwWmet0TOTe2PSiJuMntExoshbm1kh2wQzzzAA';
     $expected_placeholder_replacement = '<script type="application/vnd.drupal-ajax" data-big-pipe-replacement-for-placeholder-with-id="' . $big_pipe_placeholder_id . '">';
     $this->assertRaw('The count is 1.');
     $this->assertNoRaw('The count is 2.');
diff --git a/core/tests/Drupal/Tests/Component/Utility/UrlHelperTest.php b/core/tests/Drupal/Tests/Component/Utility/UrlHelperTest.php
index 41d1739263e..17e0dd3db08 100644
--- a/core/tests/Drupal/Tests/Component/Utility/UrlHelperTest.php
+++ b/core/tests/Drupal/Tests/Component/Utility/UrlHelperTest.php
@@ -22,7 +22,7 @@ class UrlHelperTest extends UnitTestCase {
       [['a' => ' &#//+%20@۞'], 'a=%20%26%23//%2B%2520%40%DB%9E', 'Value was properly encoded.'],
       [[' &#//+%20@۞' => 'a'], '%20%26%23%2F%2F%2B%2520%40%DB%9E=a', 'Key was properly encoded.'],
       [['a' => '1', 'b' => '2', 'c' => '3'], 'a=1&b=2&c=3', 'Multiple values were properly concatenated.'],
-      [['a' => ['b' => '2', 'c' => '3'], 'd' => 'foo'], 'a[b]=2&a[c]=3&d=foo', 'Nested array was properly encoded.'],
+      [['a' => ['b' => '2', 'c' => '3'], 'd' => 'foo'], 'a%5Bb%5D=2&a%5Bc%5D=3&d=foo', 'Nested array was properly encoded.'],
       [['foo' => NULL], 'foo', 'Simple parameters are properly added.'],
     ];
   }