.../EntityResource/Tour/TourXmlAnonTest.php        | 26 ++++++++++++++++
 .../EntityResource/Tour/TourXmlBasicAuthTest.php   | 36 ++++++++++++++++++++++
 .../EntityResource/Tour/TourXmlCookieTest.php      | 31 +++++++++++++++++++
 .../src/Functional/XmlNormalizationQuirksTrait.php | 13 ++++++--
 4 files changed, 103 insertions(+), 3 deletions(-)

diff --git a/core/modules/rest/tests/src/Functional/EntityResource/Tour/TourXmlAnonTest.php b/core/modules/rest/tests/src/Functional/EntityResource/Tour/TourXmlAnonTest.php
new file mode 100644
index 0000000..4c96e37
--- /dev/null
+++ b/core/modules/rest/tests/src/Functional/EntityResource/Tour/TourXmlAnonTest.php
@@ -0,0 +1,26 @@
+<?php
+
+namespace Drupal\Tests\rest\Functional\EntityResource\Tour;
+
+use Drupal\Tests\rest\Functional\AnonResourceTestTrait;
+use Drupal\Tests\rest\Functional\EntityResource\XmlEntityNormalizationQuirksTrait;
+
+/**
+ * @group rest
+ */
+class TourXmlAnonTest extends TourResourceTestBase {
+
+  use AnonResourceTestTrait;
+  use XmlEntityNormalizationQuirksTrait;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $format = 'xml';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $mimeType = 'text/xml; charset=UTF-8';
+
+}
diff --git a/core/modules/rest/tests/src/Functional/EntityResource/Tour/TourXmlBasicAuthTest.php b/core/modules/rest/tests/src/Functional/EntityResource/Tour/TourXmlBasicAuthTest.php
new file mode 100644
index 0000000..1f1860b
--- /dev/null
+++ b/core/modules/rest/tests/src/Functional/EntityResource/Tour/TourXmlBasicAuthTest.php
@@ -0,0 +1,36 @@
+<?php
+
+namespace Drupal\Tests\rest\Functional\EntityResource\Tour;
+
+use Drupal\Tests\rest\Functional\BasicAuthResourceTestTrait;
+use Drupal\Tests\rest\Functional\EntityResource\XmlEntityNormalizationQuirksTrait;
+
+/**
+ * @group rest
+ */
+class TourXmlBasicAuthTest extends TourResourceTestBase {
+
+  use BasicAuthResourceTestTrait;
+  use XmlEntityNormalizationQuirksTrait;
+
+  /**
+   * {@inheritdoc}
+   */
+  public static $modules = ['basic_auth'];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $format = 'xml';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $mimeType = 'text/xml; charset=UTF-8';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $auth = 'basic_auth';
+
+}
diff --git a/core/modules/rest/tests/src/Functional/EntityResource/Tour/TourXmlCookieTest.php b/core/modules/rest/tests/src/Functional/EntityResource/Tour/TourXmlCookieTest.php
new file mode 100644
index 0000000..10845c1
--- /dev/null
+++ b/core/modules/rest/tests/src/Functional/EntityResource/Tour/TourXmlCookieTest.php
@@ -0,0 +1,31 @@
+<?php
+
+namespace Drupal\Tests\rest\Functional\EntityResource\Tour;
+
+use Drupal\Tests\rest\Functional\CookieResourceTestTrait;
+use Drupal\Tests\rest\Functional\EntityResource\XmlEntityNormalizationQuirksTrait;
+
+/**
+ * @group rest
+ */
+class TourXmlCookieTest extends TourResourceTestBase {
+
+  use CookieResourceTestTrait;
+  use XmlEntityNormalizationQuirksTrait;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $format = 'xml';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $mimeType = 'text/xml; charset=UTF-8';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $auth = 'cookie';
+
+}
diff --git a/core/modules/rest/tests/src/Functional/XmlNormalizationQuirksTrait.php b/core/modules/rest/tests/src/Functional/XmlNormalizationQuirksTrait.php
index d3b2687..3dee9d4 100644
--- a/core/modules/rest/tests/src/Functional/XmlNormalizationQuirksTrait.php
+++ b/core/modules/rest/tests/src/Functional/XmlNormalizationQuirksTrait.php
@@ -12,6 +12,8 @@
    *
    * The XML encoding:
    * - maps empty arrays to the empty string
+   * - maps single-item arrays to just that single item
+   * - restructures multiple-item arrays that lives in a single-item array
    *
    * @param array $normalization
    *   A normalization.
@@ -31,12 +33,17 @@ protected function applyXmlDecodingQuirks(array $normalization) {
         if (count($value) === 1 && is_numeric(array_keys($value)[0]) && is_scalar($value[0])) {
           $value = $value[0];
         }
-        // Restructure multiple-item numeric arrays.
+        // Restructure multiple-item arrays inside a single-item numeric array.
         // @see \Symfony\Component\Serializer\Encoder\XmlEncoder::buildXml()
-        elseif (is_numeric(array_keys($value)[0]) && is_array(reset($value)) && is_numeric(array_keys(reset($value))[0])) {
+        elseif (is_numeric(array_keys($value)[0]) && is_array(reset($value))) {
           $rewritten_value = [];
           foreach ($value[0] as $child_key => $child_value) {
-            $rewritten_value[$child_key] = ['@key' => $child_key ] + $child_value;
+            if (is_numeric(array_keys(reset($value))[0])) {
+              $rewritten_value[$child_key] = ['@key' => $child_key ] + $child_value;
+            }
+            else {
+              $rewritten_value[$child_key] = $child_value;
+            }
           }
           $value = $rewritten_value;
         }