Get every post with value in meta key

Try this:

$meta_query_args = array(
    'meta_query'=>array(
        array(
            'key' => '_cornerstone_data',
            'value' => $attachment_url,
            'compare' => 'LIKE',
        ),
    )
);

$meta_query = new WP_Query( $meta_query_args );

$posts = $meta_query->posts;

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