Update to my earlier post - I just discovered that all shippers are listed in the email to customer even when NO tracking number is uploaded, just a comment is made OR order status is changed. On the other hand, when the Append Comments checkbox is UNchecked, no shippers are listed even when a tracking number was added. (The last one might be the expected behavior for all I know, I'm just describing my observations; perhaps shipper info is regarded as part of the comment.)
Hope this can help find a solution...
Thanks,
Magz