Mail dropped after opendkim "failed to parse authentication-results: header field"

Looking into this a bit further, it looks like postfix delivers the mail to the spam filter, and the output of the spam filter is “action discard”. From this log it is not clear which spam action is applied. Perhaps sudo journalctl -u spampd has info.
Otherwise, look at this post to enable some spampd logging. It might provide additional info the next time such a mail arrives.

Ah, which modifications are these?