diff -u b/core/lib/Drupal/Component/Utility/UrlHelper.php b/core/lib/Drupal/Component/Utility/UrlHelper.php
--- b/core/lib/Drupal/Component/Utility/UrlHelper.php
+++ b/core/lib/Drupal/Component/Utility/UrlHelper.php
@@ -201,12 +201,14 @@
     $parsed = [];
     foreach (explode('&', $query) as $param) {
       list($name) = explode('=', $param, 2);
+      $name = rawurldecode($name);
+      // Extract the first item of a potentially nested parameter name.
       $name = strstr($name, '[', true) ?: $name;
       // Parse the query parameter.
       parse_str($param, $param);
       // Merge the parameter into the parsed array.
       if (isset($parsed[$name])) {
-        $parsed[$name] = array_merge($parsed[$name], reset($param));
+        $parsed[$name] = array_merge_recursive($parsed[$name], reset($param));
       }
       else {
         $parsed[$name] = reset($param);
diff -u b/core/lib/Drupal/Core/Url.php b/core/lib/Drupal/Core/Url.php
--- b/core/lib/Drupal/Core/Url.php
+++ b/core/lib/Drupal/Core/Url.php
@@ -295,15 +295,15 @@
     }
 
     if (!empty($uri_parts['query'])) {
-      $uri_query = UrlHelper::parseQueryString($uri_parts['query']);
+      $uri_query = [];
+      parse_str($uri_parts['query'], $uri_query);
       $uri_options['query'] = isset($uri_options['query']) ? $uri_options['query'] + $uri_query : $uri_query;
       unset($uri_parts['query']);
     }
     }
 
     if (!empty($uri_parts['query'])) {
-      $uri_query = [];
-      parse_str($uri_parts['query'], $uri_query);
+      $uri_query = UrlHelper::parseQueryString($uri_parts['query']);
       $uri_options['query'] = isset($uri_options['query']) ? $uri_options['query'] + $uri_query : $uri_query;
       unset($uri_parts['query']);
     }