WP_Query custom order and pagination

I am seeing some strange behaviour when combining a custom order with pagination when using WP_Query. The arguments for my query are as follows:

$args = array(
    'post_type'       => 'news',
    'posts_per_page'  => 10,
    'meta_key'        => 'news_date',
    'orderby'         => 'meta_value_num',
    'order'           => 'DESC',
    'paged'           => $my_page
);

The variable $my_page contains the current page number as extracted from the URL or set to 1 if not defined in the URL.

This query returns the correct number of results but the order is incorrect. Removing the paged parameter results in the correct order being achieved but obviously breaks my pagination.

Is there something I am doing wrong? Thanks.