diff -u b/core/modules/media_library/src/Form/AddFormBase.php b/core/modules/media_library/src/Form/AddFormBase.php
--- b/core/modules/media_library/src/Form/AddFormBase.php
+++ b/core/modules/media_library/src/Form/AddFormBase.php
@@ -138,7 +138,7 @@
       'js-media-library-add-form',
     ];
 
-    $added_media = $form_state->get('media');
+    $added_media = $this->getAddedMediaItems($form_state);
     if (empty($added_media)) {
       $form['#attributes']['class'][] = 'media-library-add-form--without-input';
       $form = $this->buildInputElement($form, $form_state);
@@ -474,8 +474,7 @@
    * {@inheritdoc}
    */
   public function validateForm(array &$form, FormStateInterface $form_state) {
-    $added_media = $form_state->get('media') ?: [];
-    foreach ($added_media as $delta => $media) {
+    foreach ($this->getAddedMediaItems($form_state) as $delta => $media) {
       $this->validateMediaEntity($media, $form, $form_state, $delta);
     }
   }
@@ -502,8 +501,7 @@
    * {@inheritdoc}
    */
   public function submitForm(array &$form, FormStateInterface $form_state) {
-    $added_media = $form_state->get('media') ?: [];
-    foreach ($added_media as $delta => $media) {
+    foreach ($this->getAddedMediaItems($form_state) as $delta => $media) {
       EntityFormDisplay::collectRenderDisplay($media, 'media_library')
         ->extractFormValues($media, $form['media'][$delta]['fields'], $form_state);
       $this->prepareMediaEntityForSave($media);
@@ -528,10 +526,9 @@
       return $form;
     }
 
-    $added_media = $form_state->get('media') ?: [];
     $media_ids = array_map(function (MediaInterface $media) {
       return $media->id();
-    }, $added_media);
+    }, $this->getAddedMediaItems($form_state));
 
     // Get the render array for the media library. The media library state might
     // contain the 'media_library_content' when it has been opened from a
@@ -599,7 +596,7 @@
   }
 
   /**
-   * Get all pre-selected media IDs from the form state.
+   * Get all pre-selected media items from the form state.
    *
    * @param \Drupal\Core\Form\FormStateInterface $form_state
    *   The current form state.
@@ -616,15 +613,28 @@
   }
 
   /**
-   * Get all pre-selected and added media IDs from the form state.
+   * Get all added media items from the form state.
+   *
+   * @param \Drupal\Core\Form\FormStateInterface $form_state
+   *   The current form state.
+   *
+   * @return \Drupal\media\MediaInterface[]
+   *   An array containing the added media items keyed by delta. The media items
+   *   won't have an ID untill they are saved in ::submitForm().
+   */
+  protected function getAddedMediaItems(FormStateInterface $form_state) {
+    return $form_state->get('media') ?: [];
+  }
+
+  /**
+   * Get all pre-selected and added media items from the form state.
    *
    * @param \Drupal\Core\Form\FormStateInterface $form_state
    *   The current form state.
    *
    * @return \Drupal\media\MediaInterface[]
-   *   An array containing all pre-selected and added media items. The
-   *   pre-selected media items are keyed by ID and the added items are keyed by
-   *   delta.
+   *   An array containing all pre-selected and added media items with
+   *   renumbered numeric keys.
    */
   protected function getCurrentMediaItems(FormStateInterface $form_state) {
     $pre_selected_media = $this->getPreSelectedMediaItems($form_state);
diff -u b/core/modules/media_library/tests/src/FunctionalJavascript/MediaLibraryTest.php b/core/modules/media_library/tests/src/FunctionalJavascript/MediaLibraryTest.php
--- b/core/modules/media_library/tests/src/FunctionalJavascript/MediaLibraryTest.php
+++ b/core/modules/media_library/tests/src/FunctionalJavascript/MediaLibraryTest.php
@@ -602,7 +602,7 @@
     $assert_session->elementNotExists('css', '.media-library-add-form--with-input');
     $page->attachFileToField('Add files', $this->container->get('file_system')->realpath($png_image->uri));
     $assert_session->assertWaitOnAjaxRequest();
-    $assert_session->pageTextContains('The media item has been created. Fill in any required fields and save to make it available in the media library.');
+    $assert_session->pageTextContains('The media item has been created. Fill in any required fields and save to add it to the media library.');
     $assert_session->elementExists('css', '.media-library-add-form--with-input');
     $assert_session->elementNotExists('css', '.media-library-add-form--without-input');
     // We do not have a pre-selected items, so the container should not be added