[exim] match_domain won't work in ACL section

Greetings,

I am trying to set up email message checking for allowed email
addresses combination. I noticeed I have trouble trying to check
whether a header field matches certain criteria.

Example. I have the following lines in acl_smtp_data:

warn    add_header = X-ACL-Exim-To: $header_to:
          condition  = ${if
match_domain{$header_to:}{/etc/exim/acl/a:/etc/exim/acl/b}}
          add_header = X-ACL-Exim-Blocked: true

The /etc/exim/acl/a:

example.com

The /etc/exim/acl/b:

example.net

When I send a message to

user@???

only the first header is added (the one added unconditionally). What
is wrong with match_domain condition? How can I see why the matching
doesn't work?

Exim version: 4.66

Thanks.