Hi,
I suppose that you're using the status changing popup of the orders listing because I was only able to replicate it that way. It worked fine with the order status changing system of the order details page or when a payment plugin was automatically changing the status.
Add the line:
<input type="hidden" name="data[order][mail][reply_email]" value="<?php echo $this->element->mail->reply_email;?>" />
at the end of the file "mailform" via the menu Display>Views and that will make it work for the order listing order status changing popup too.
We'll add that on our end for the next version of HikaShop.