diff --git a/src/wp-includes/comment.php b/src/wp-includes/comment.php
index c015fea4f2..9054e1acee 100644
--- a/src/wp-includes/comment.php
+++ b/src/wp-includes/comment.php
@@ -423,7 +423,11 @@ function get_comment_count( $post_id = 0 ) {
  * @return int|bool Meta ID on success, false on failure.
  */
 function add_comment_meta($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(), 'comment' );
+	}
+	return $added;
 }
 
 /**
@@ -442,7 +446,11 @@ function add_comment_meta($comment_id, $meta_key, $meta_value, $unique = false)
  * @return bool True on success, false on failure.
  */
 function delete_comment_meta($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(), 'comment' );
+	}
+	return $deleted;
 }
 
 /**
@@ -479,7 +487,11 @@ function get_comment_meta($comment_id, $key = '', $single = false) {
  * @return int|bool Meta ID if the key didn't exist, true on successful update, false on failure.
  */
 function update_comment_meta($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(), 'comment' );
+	}
+	return $updated;
 }
 
 /**
diff --git a/tests/phpunit/tests/comment/query.php b/tests/phpunit/tests/comment/query.php
index dbfe8dfdee..4d16b29675 100644
--- a/tests/phpunit/tests/comment/query.php
+++ b/tests/phpunit/tests/comment/query.php
@@ -3025,4 +3025,139 @@ class Tests_Comment_Query extends WP_UnitTestCase {
 
 		$this->assertEquals( $number_of_queries, $wpdb->num_queries );
 	}
+
+	/**
+	 * @ticket 40669
+	 */
+	public function test_add_comment_meta_should_invalidate_query_cache() {
+		global $wpdb;
+
+		$p = self::factory()->post->create( array( 'post_status' => 'publish' ) );
+		$c1 = self::factory()->comment->create_post_comments( $p, 1 );
+		$c2 = self::factory()->comment->create_post_comments( $p, 1 );
+
+		foreach ( $c1 as $cid ) {
+			add_comment_meta( $cid, 'sauce', 'fire' );
+		}
+
+		$cached = get_comments( array(
+			'post_id' => $p,
+			'fields' => 'ids',
+			'meta_query' => array(
+				array(
+					'key' => 'sauce',
+					'value' => 'fire',
+				),
+			)
+		) );
+
+		$this->assertEqualSets( $c1, $cached );
+
+		foreach ( $c2 as $cid ) {
+			add_comment_meta( $cid, 'sauce', 'fire' );
+		}
+
+		$found = get_comments( array(
+			'post_id' => $p,
+			'fields' => 'ids',
+			'meta_query' => array(
+				array(
+					'key' => 'sauce',
+					'value' => 'fire',
+				),
+			)
+		) );
+
+		$this->assertEqualSets( array_merge( $c1, $c2 ), $found );
+	}
+
+	/**
+	 * @ticket 40669
+	 */
+	public function test_update_comment_meta_should_invalidate_query_cache() {
+		global $wpdb;
+
+		$p = self::factory()->post->create( array( 'post_status' => 'publish' ) );
+		$c1 = self::factory()->comment->create_post_comments( $p, 1 );
+		$c2 = self::factory()->comment->create_post_comments( $p, 1 );
+
+		foreach ( array_merge( $c1, $c2 ) as $cid ) {
+			add_comment_meta( $cid, 'sauce', 'fire' );
+		}
+
+		$cached = get_comments( array(
+			'post_id' => $p,
+			'fields' => 'ids',
+			'meta_query' => array(
+				array(
+					'key' => 'sauce',
+					'value' => 'fire',
+				),
+			)
+		) );
+
+		$this->assertEqualSets( array_merge( $c1, $c2 ), $cached );
+
+		foreach ( $c2 as $cid ) {
+			update_comment_meta( $cid, 'sauce', 'foo' );
+		}
+
+		$found = get_comments( array(
+			'post_id' => $p,
+			'fields' => 'ids',
+			'meta_query' => array(
+				array(
+					'key' => 'sauce',
+					'value' => 'fire',
+				),
+			)
+		) );
+
+		$this->assertEqualSets( $c1, $found );
+	}
+
+	/**
+	 * @ticket 40669
+	 */
+	public function test_delete_comment_meta_should_invalidate_query_cache() {
+		global $wpdb;
+
+		$p = self::factory()->post->create( array( 'post_status' => 'publish' ) );
+		$c1 = self::factory()->comment->create_post_comments( $p, 1 );
+		$c2 = self::factory()->comment->create_post_comments( $p, 1 );
+
+		foreach ( array_merge( $c1, $c2 ) as $cid ) {
+			add_comment_meta( $cid, 'sauce', 'fire' );
+		}
+
+		$cached = get_comments( array(
+			'post_id' => $p,
+			'fields' => 'ids',
+			'meta_query' => array(
+				array(
+					'key' => 'sauce',
+					'value' => 'fire',
+				),
+			)
+		) );
+
+		$this->assertEqualSets( array_merge( $c1, $c2 ), $cached );
+
+		foreach ( $c2 as $cid ) {
+			delete_comment_meta( $cid, 'sauce' );
+		}
+
+		$found = get_comments( array(
+			'post_id' => $p,
+			'fields' => 'ids',
+			'meta_query' => array(
+				array(
+					'key' => 'sauce',
+					'value' => 'fire',
+				),
+			)
+		) );
+
+		$this->assertEqualSets( $c1, $found );
+	}
 }