reverted:
--- b/core/modules/comment/comment.module
+++ a/core/modules/comment/comment.module
@@ -503,14 +503,14 @@
     $preview_build['comment_preview'] = $comment_build;
   }
 
-  $build = [];
   if ($comment->hasParentComment()) {
+    $build = [];
     $parent = $comment->getParentComment();
     if ($parent && $parent->isPublished()) {
       $build = \Drupal::entityTypeManager()->getViewBuilder('comment')->view($parent);
     }
   }
+  else {
-  elseif ($entity) {
     // The comment field output includes rendering the parent entity of the
     // thread to which the comment is a reply. The rendered entity output
     // includes the comment reply form, which contains the comment preview and
@@ -607,8 +607,9 @@
 
   $variables['submitted'] = t('Submitted by @username on @datetime', ['@username' => $variables['author'], '@datetime' => $variables['created']]);
 
+  if ($comment->hasParentComment()) {
-  if ($comment_parent = $comment->getParentComment()) {
     // Fetch and store the parent comment information for use in templates.
+    $comment_parent = $comment->getParentComment();
     $account_parent = $comment_parent->getOwner();
     $variables['parent_comment'] = $comment_parent;
     $username = [
reverted:
--- b/core/modules/comment/comment.tokens.inc
+++ a/core/modules/comment/comment.tokens.inc
@@ -124,11 +124,13 @@
   $token_service = \Drupal::token();
 
   $url_options = ['absolute' => TRUE];
-  $langcode = NULL;
   if (isset($options['langcode'])) {
     $url_options['language'] = \Drupal::languageManager()->getLanguage($options['langcode']);
     $langcode = $options['langcode'];
   }
+  else {
+    $langcode = NULL;
+  }
   $replacements = [];
 
   if ($type == 'comment' && !empty($data['comment'])) {
@@ -198,11 +200,9 @@
 
         case 'parent':
           if ($comment->hasParentComment()) {
+            $parent = $comment->getParentComment();
+            $bubbleable_metadata->addCacheableDependency($parent);
+            $replacements[$original] = $parent->getSubject();
-            $replacements[$original] = '[not found]';
-            if ($parent = $comment->getParentComment()) {
-              $bubbleable_metadata->addCacheableDependency($parent);
-              $replacements[$original] = $parent->getSubject();
-            }
           }
           break;
 
@@ -219,12 +219,10 @@
           break;
 
         case 'entity':
+          $entity = $comment->getCommentedEntity();
+          $bubbleable_metadata->addCacheableDependency($entity);
+          $title = $entity->label();
+          $replacements[$original] = $title;
-          $replacements[$original] = '[not found]';
-          if ($entity = $comment->getCommentedEntity()) {
-            $bubbleable_metadata->addCacheableDependency($entity);
-            $title = $entity->label();
-            $replacements[$original] = $title;
-          }
           break;
       }
     }
reverted:
--- b/core/modules/comment/src/CommentInterface.php
+++ a/core/modules/comment/src/CommentInterface.php
@@ -40,10 +40,6 @@
   /**
    * Determines if this comment is a reply to another comment.
    *
-   * Calling getParentComment() without checking hasParentComment() first, is
-   * allowed. hasParentComment() is generally faster but can still return a
-   * value even when the parent comment cannot be loaded.
-   *
    * @return bool
    *   TRUE if the comment has a parent comment otherwise FALSE.
    */
@@ -53,8 +49,7 @@
    * Returns the parent comment entity if this is a reply to a comment.
    *
    * @return \Drupal\comment\CommentInterface|null
+   *   A comment entity of the parent comment or NULL if there is no parent.
-   *   A comment entity of the parent comment; NULL if there is no parent or
-   *   if the parent somehow cannot be loaded.
    */
   public function getParentComment();
 
@@ -70,10 +65,6 @@
   /**
    * Returns the ID of the entity to which the comment is attached.
    *
-   * Calling getCommentedEntity() without checking getCommentedEntityId() first,
-   * is allowed. getCommentedEntityId() is generally faster but can still return
-   * a value even when the commented entity cannot be loaded.
-   *
    * @return int
    *   The ID of the entity to which the comment is attached.
    */
reverted:
--- b/core/modules/comment/src/CommentViewBuilder.php
+++ a/core/modules/comment/src/CommentViewBuilder.php
@@ -80,11 +80,9 @@
 
     /** @var \Drupal\comment\CommentInterface $entity */
     // Store a threading field setting to use later in self::buildComponents().
+    $build['#comment_threaded'] = $entity->getCommentedEntity()
+      ->getFieldDefinition($entity->getFieldName())
+      ->getSetting('default_mode') === CommentManagerInterface::COMMENT_MODE_THREADED;
-    $commented_entity = $entity->getCommentedEntity();
-    $build['#comment_threaded'] =
-      !$commented_entity
-      || $commented_entity->getFieldDefinition($entity->getFieldName())
-        ->getSetting('default_mode') === CommentManagerInterface::COMMENT_MODE_THREADED;
     // If threading is enabled, don't render cache individual comments, but do
     // keep the cacheability metadata, so it can bubble up.
     if ($build['#comment_threaded']) {
@@ -144,7 +142,8 @@
       $commented_entity = $entity->getCommentedEntity();
 
       $build[$id]['#entity'] = $entity;
+      $build[$id]['#theme'] = 'comment__' . $entity->getFieldName() . '__' . $commented_entity->bundle();
+
-      $build[$id]['#theme'] = 'comment__' . $entity->getFieldName() . '__' . ($commented_entity ? $commented_entity->bundle() : '');
       $display = $displays[$entity->bundle()];
       if ($display->getComponent('links')) {
         $build[$id]['links'] = [
@@ -165,8 +164,7 @@
         $build[$id]['#attached'] = [];
       }
       $build[$id]['#attached']['library'][] = 'comment/drupal.comment-by-viewer';
+      if ($attach_history && $commented_entity->getEntityTypeId() === 'node') {
-      if ($attach_history && $commented_entity
-        && $commented_entity->getEntityTypeId() === 'node') {
         $build[$id]['#attached']['library'][] = 'comment/drupal.comment-new-indicator';
 
         // Embed the metadata for the comment "new" indicators on this node.
reverted:
--- b/core/modules/comment/src/Entity/Comment.php
+++ a/core/modules/comment/src/Entity/Comment.php
@@ -203,8 +203,8 @@
    */
   public function referencedEntities() {
     $referenced_entities = parent::referencedEntities();
+    if ($this->getCommentedEntityId()) {
+      $referenced_entities[] = $this->getCommentedEntity();
-    if ($commented_entity = $this->getCommentedEntity()) {
-      $referenced_entities[] = $commented_entity;
     }
     return $referenced_entities;
   }