Sub-menu Hover problem when create Child Theme

Hello I think this is a bug.
In lasted version(3.5.1) when i create a child theme and activate child them on rtl languages , i see gray like below main menu when i hover it.
and check out CSS and find out the problem is in inline CSS and it’s be OK if we delete
background-color: #f0f0f0;

CSS class:

.main-navigation ul.menu ul.sub-menu,.main-navigation ul.nav-menu ul.children {
    /*! background-color: #f0f0f0; */
}

about my child theme: i copy storefront default style-rt.css file to theme folder(child theme)

my style.css file in storefront-child

/*!
Theme Name:   Storefront Child
Theme URI:    https://woocommerce.com/storefront/
Author:       Automattic
Author URI:   https://woocommerce.com/
Description:  Storefront is the perfect theme for your next WooCommerce project. Designed and developed by WooCommerce Core developers, it features a bespoke integration with WooCommerce itself plus many of the most popular customer facing WooCommerce extensions. There are several layout & color options to personalise your shop, multiple widget regions, a responsive design and much more. Developers will love its lean and extensible codebase making it a joy to customize and extend. Looking for a WooCommerce theme? Look no further!
Version:      3.5.1
Tested up to: 5.6.0
Requires PHP: 5.6.0
Template: storefront
Text Domain: storefront-child
License:      GNU General Public License v2 or later
License URI:  https://www.gnu.org/licenses/gpl-2.0.html
Tags:         e-commerce, two-columns, left-sidebar, right-sidebar, custom-background, custom-colors, custom-header, custom-menu, featured-images, full-width-template, threaded-comments, accessibility-ready, rtl-language-support, footer-widgets, sticky-post, theme-options, editor-style
*/
/*!
 * normalize.css v3.0.3 | MIT License | github.com/necolas/normalize.css
 */

function.php file

<?php
add_action( 'wp_enqueue_scripts', 'enqueue_parent_styles' );
function enqueue_parent_styles() {
    wp_enqueue_style( 'parent-style-copied', get_template_directory_uri().'/style.css' );
    wp_enqueue_style( 'parent-style-rtl', get_template_directory_uri().'/style-rtl.css' );
}
?>