rspamd with opensmtpd-filter-rspamd, missing X-Spam-Symbols

I recently moved from

action "mda_with_aliases" mda \
"/usr/local/bin/rspamc --mime --exec '/usr/local/libexec/dovecot/dovecot-lda -f %{sender} -d %{dest:strip} -a %{dest}'" \
alias <aliases> user vmail

to

filter rspamd proc-exec "filter-rspamd"

and

action "mda_with_aliases" mda \
"/usr/local/libexec/dovecot/dovecot-lda -f '%{sender}' -d '%{dest:strip}' -a '%{dest}'" \
alias <aliases> user vmail


I used to get a bunch of X-Spam-* header lines telling me why a mail got classified as spam / ham, e.g.:

X-Spam-Scanner: rspamc 1.9.4
X-Spam-Scan-Time: 0.308
X-Spam: yes
X-Spam-Action: add header
X-Spam-Score: 6.00 / 15.00
X-Spam-Level: ******
X-Spam-Symbols: ARC_NA,
FROM_HAS_DN,
MISSING_MIME_VERSION,
R_MISSING_CHARSET,
DMARC_NA,
BROKEN_CONTENT_TYPE,
R_SPF_SOFTFAIL,
RCPT_COUNT_ONE,
RCVD_COUNT_THREE,
TO_DN_ALL,
FROM_EQ_ENVFROM,
R_DKIM_NA,
MIME_TRACE,
ASN,
RCVD_TLS_LAST,
TO_DOM_EQ_FROM_DOM

After the change I only get

X-Spam-Action: add header
X-Spam: yes
X-Spam-Score: 6.168947 / 6

And I need to wade through the rspamd.log to find out what it did.

I tried

extended_spam_headers = true;

in /etc/rspamd/override.d/milter_headers.conf
or
/etc/rspamd/local.d/milter_headers.conf

But that doesn't seem to do anything?

The documentation has this to say:

"The milter headers module (formerly known as rmilter headers) has
been added in Rspamd 1.5 to provide a relatively simple way to
configure adding/removing of headers via Rmilter
(the alternative being to use the API)."
^^^^^^^^^^^^^^^ ^^^^^^^^^^^

So maybe opensmtpd-filter-rspamd using the API and I have to push a
different button?

Any insight would be appreciated.
Thanks,
Florian

--
I'm not entirely sure you are real.