[exim] "headers_add" working differently?

Hello,

we have some forwarding routers in our exim.conf.
The used to leave a tag upon execution as
"Delivered-To"-Header.

Thus, a typical mail could read like

    [...]
    Delivered-To: postmaster@???
    Delivered-To: postmaster@???
    Delivered-To: marian.eichholz@???
    [...]

(taken from an exim 4.43 delivery)

In recent mails, there is only the one line present:

        Delivered-To: marian.eichholz@???

That's odd, because the routing in the respective box did
not change (basically) in the meantime.

Furthermore, in a test session debugging output, I find
three references to the added header lines, but only one
line in the actual delivered mail:

Of course, we run 4.51 :-)

    13:47:14 27307 added header line(s):
    13:47:14 27307 Delivered-To: marian.eichholz@???
    --
    13:47:14 27308 added header line(s):
    13:47:14 27308 Delivered-To: postmaster@???
    --
    13:47:14 27320 added header line(s):
    13:47:14 27320 Delivered-To: postmaster@???

So my question is, if anything seriously changed in
"header_add" that can cause such trouble?

The router bits read like this, nothing special:

    d_domainalias:
      driver = redirect
      allow_defer
      allow_fail
      condition = ...
      data = ...
      headers_add = "Delivered-To: $local_part@$domain"
      retry_use_local_part

    [...]

    d_forward:
      driver = redirect
      allow_defer
      allow_fail 
      data = ...
      forbid_file
      forbid_pipe
      headers_add = "Delivered-To: $local_part@$domain"
      retry_use_local_part

Thanks in advance!

Yours,

Marian