That's proper behavior. BCC is *blind* carbon copy, specifically
intended to *not* show who else got copies. I'd argue that the only
time it's acceptable to list recipients there is if you are feeding
to an MSA that's like 'sendmail -t' that needs it to get additional
recipients because you can't speak SMTP and hand off RCPT TO:<...> for
the Bcc people.
To get back to the larger point ... with sendmail/pipe, you are accepting
reduced functionality. MH-style BCC will continue to work. But Dcc
(which behaves like everyone other MUA's Bcc) will not.
--Ken