Index: src/wp-login.php
===================================================================
--- src/wp-login.php	(revision 43269)
+++ src/wp-login.php	(working copy)
@@ -543,6 +543,19 @@
 		}
 
 		/**
+		 * Removes wp-postpass_ cookie on user logout
+		 *
+		 * @since 4.9.7
+		 *
+		 */
+		if ( isset( $_COOKIE[ 'wp-postpass_' . COOKIEHASH ] ) ) {
+
+			unset( $_COOKIE[ 'wp-postpass_' . COOKIEHASH ] );
+  			setcookie( 'wp-postpass_' . COOKIEHASH, '', time() - ( 15 * 60 ) );
+
+		}
+
+		/**
 		 * Filters the log out redirect URL.
 		 *
 		 * @since 4.2.0