Index: src/wp-includes/comment.php
===================================================================
--- src/wp-includes/comment.php	(revision 41324)
+++ src/wp-includes/comment.php	(working copy)
@@ -423,7 +423,12 @@
  * @return int|bool Meta ID on success, false on failure.
  */
 function add_comment_meta($comment_id, $meta_key, $meta_value, $unique = false) {
-	return add_metadata('comment', $comment_id, $meta_key, $meta_value, $unique);
+	$added = add_metadata( 'comment', $comment_id, $meta_key, $meta_value, $unique );
+	if ( $added ) {
+		wp_cache_set( 'last_changed', microtime(), 'comments' );
+	}
+
+	return $added;
 }
 
 /**
@@ -442,7 +447,12 @@
  * @return bool True on success, false on failure.
  */
 function delete_comment_meta($comment_id, $meta_key, $meta_value = '') {
-	return delete_metadata('comment', $comment_id, $meta_key, $meta_value);
+	$deleted = delete_metadata( 'comment', $comment_id, $meta_key, $meta_value );
+	if ( $deleted ) {
+		wp_cache_set( 'last_changed', microtime(), 'comments' );
+	}
+
+	return $deleted;
 }
 
 /**
@@ -479,7 +489,12 @@
  * @return int|bool Meta ID if the key didn't exist, true on successful update, false on failure.
  */
 function update_comment_meta($comment_id, $meta_key, $meta_value, $prev_value = '') {
-	return update_metadata('comment', $comment_id, $meta_key, $meta_value, $prev_value);
+	$updated = update_metadata( 'comment', $comment_id, $meta_key, $meta_value, $prev_value );
+	if ( $updated ) {
+		wp_cache_set( 'last_changed', microtime(), 'comments' );
+	}
+
+	return $updated;
 }
 
 /**