I've been using the following snippet without any issues. For no reason that I can think of it stopped being triggered today.
Could it perhaps be written better?
add_action('woocommerce_order_status_completed', 'email_completed_order_admin_notification', 10, 2 );
function email_completed_order_admin_notification( $order_id, $order ) {
WC()->mailer()->get_emails()['WC_Email_New_Order']->trigger( $order_id );
}