Answer by Marc for Why does Simplepie return feed items in a wrong order?

The simplest fix for someone who is experiencing this issue is this:

function my_add_force_rss($feed,$url){
   $feed->force_feed(true);
   $feed->enable_order_by_date(false);
}
add_action('wp_feed_options', 'my_add_force_rss', 10,2);

Placing this in your functions.php will force any feed to be displayed in the order that it is received.