This reply built in mail/order_confirm.tpl template
And variables are built in storefront/model/checkout/order.php
You need to look for:
$template->data['title'] = sprintf($language->get('text_subject'), html_entity_decode($order_row['store_name'], ENT_QUOTES,
'UTF-8'), $order_id);
Basically, you need to set your store name correctly in configuration.