[exim] why multiple recipients in Envelope-To header?

Hello,

in my transport where the final delivery to the mailbox is done, I have
set the option "envelope_to_add". So I even if the recipient in the
To:-header is faked, I can see to which address the email was sent to
within my mailserver.

But recently some users pointed out, that this doesn't behave 100%
correctly (at least in my understanding of this option): If one email is
delivered to e.g. 3 addresses by blind copies (bcc-header), then each
recipient should only see his address, that means the one and only
address why it has been delivered to his mailbox. But instead, each
recipient sees _all_ blind copy recipient, so in fact it isn't a blind
copy any more. And this doesn't only happen when the message is being
delivered to the same mailbox. It also happens with a delivery is done
for completely different users/mailboxes. This looks like this:

Envelope-To: first@???, second@???, third@???

But from what I expected this option to do each recipient should only see

Envelope-To: first@???
or
Envelope-To: second@???
or
Envelope-To: third@???

Is this an error within exim? Or are my expectations to this option
wrong? How can I tell exim to only show one Envelope-To address?

Regards
Marten