Do you have a header.php in the child theme? If there is this code in it then delete it.
<?php wp_enqueue_style('responsive-style', get_stylesheet_uri(), false, '1.7.9');?>
Also there should be no @import in the style.css
This issue only affected a small portion of users who had old code in the header.php. The change was made so uses are able to load a minified version of the css to increase the page load time and thus increase the speed of the site.