diff -u b/core/lib/Drupal/Core/Render/Renderer.php b/core/lib/Drupal/Core/Render/Renderer.php
--- b/core/lib/Drupal/Core/Render/Renderer.php
+++ b/core/lib/Drupal/Core/Render/Renderer.php
@@ -226,12 +226,16 @@
         return '';
       }
 
-      // Avoid making this change for the referenced elements variable because
-      // it could cause unexpected behaviour with other templating engines than
-      // Twig. Save the original referenced variable to allow making changes for
-      // the referenced variable in case explicitly wanted so.
-      $original_elements = &$elements;
+      // Remove all elements except the children because the main level has been
+      // already rendered when the #render_children is set, and therefore they
+      // shouldn't have any effect on the render children.
       $new_elements = array_intersect_key($elements, array_flip($children));
+      // Create a new variable that references the render array that was passed
+      // in. This allows the markup and cache information to be attached after
+      // rendering the new elements array.
+      $original_elements = &$elements;
+      // Change $elements to reference $new_elements. This prevents
+      // unintentional changes to the render array that was passed in.
       $elements = &$new_elements;
     }
 
@@ -535,11 +539,12 @@
 
     $elements['#markup'] = Markup::create($prefix . $elements['#children'] . $suffix);
 
-    // #attached and #markup values should be always saved to the referenced
-    // elements variable.
+    // #cache, #attached and #markup values should be always saved to the
+    // referenced elements variable to prevent re-rendering.
     if (isset($original_elements)) {
       $original_elements['#markup'] = $elements['#markup'];
       $original_elements['#attached'] = $elements['#attached'];
+      $original_elements['#cache'] = CacheableMetadata::createFromRenderArray($elements['#cache']);
     }
 
     // We've rendered this element (and its subtree!), now update the context.
diff -u b/core/modules/file/src/Tests/FileFieldValidateTest.php b/core/modules/file/src/Tests/FileFieldValidateTest.php
--- b/core/modules/file/src/Tests/FileFieldValidateTest.php
+++ b/core/modules/file/src/Tests/FileFieldValidateTest.php
@@ -71,10 +71,8 @@
     $field_name = strtolower($this->randomMachineName());
     $this->createFileField($field_name, 'node', $type_name, array(), array('required' => '1'));
 
-    // 128KB.
-    $small_file = $this->getTestFile('text', 131072);
-    // 1.2MB.
-    $large_file = $this->getTestFile('text', 1310720);
+    $small_file = $this->getTestFile('text', 131072); // 128KB.
+    $large_file = $this->getTestFile('text', 1310720); // 1.2MB
 
     // Test uploading both a large and small file with different increments.
     $sizes = array(
diff -u b/core/modules/image/src/Tests/ImageFieldValidateTest.php b/core/modules/image/src/Tests/ImageFieldValidateTest.php
--- b/core/modules/image/src/Tests/ImageFieldValidateTest.php
+++ b/core/modules/image/src/Tests/ImageFieldValidateTest.php
@@ -8,7 +8,6 @@
  * @group image
  */
 class ImageFieldValidateTest extends ImageFieldTestBase {
-
   /**
    * Test min/max resolution settings.
    */
@@ -162,14 +161,14 @@
    */
   public function testAJAXValidationMessage() {
     $field_name = strtolower($this->randomMachineName());
-    $this->createImageField($field_name, 'article');
+    $this->createImageField($field_name, 'article', ['cardinality' => -1]);
 
     $this->drupalGet('node/add/article');
     /** @var \Drupal\file\FileInterface[] $text_files */
     $text_files = $this->drupalGetTestFiles('text');
     $text_file = reset($text_files);
     $edit = array(
-      'files[' . $field_name . '_0]' => $this->container->get('file_system')->realpath($text_file->uri),
+      'files[' . $field_name . '_0][]' => $this->container->get('file_system')->realpath($text_file->uri),
       'title[0][value]' => $this->randomMachineName(),
     );
     $this->drupalPostAjaxForm(NULL, $edit, $field_name . '_0_upload_button');