woocommerce_order_status_changed
function forcesendmail ( $order_id, $old_status, $new_status ){
$order = new WC_Order($order_id);
if( $new_status == "processing" ) {
WC()->mailer()->emails['WC_Email_Customer_Processing_Order']->trigger($order_id);
}
}
add_action( 'woocommerce_order_status_changed', 'forcesendmail', 99, 3 );
Shubham Rathod