Saving a copy of incoming mail for specific recipient in Exim

I need to archive important mail incoming for specific address in case it gets accidentally deleted from email server, etc.

Either saving or using a pipe to archive it on backup machine is fine (I can rsync the backup automatically later, etc).

However, I have trouble to get system_filter working. I have configured it this way so far:

/etc/exim4/exim4.conf:

system_filter = /etc/exim4/system_filter
system_filter_user = Debian-exim
system_filter_group = Debian-exim

system_filter_directory_transport = local_copy_to_directory

# transport section

local_copy_to_directory:
    driver = appendfile
    delivery_date_add
    envelope_to_add
    return_path_add
    group = Debian-exim
    user = Debian-exim
    mode = 0660
    maildir_format = true
    create_directory = true

In /etc/exim4/system_filter:

# Exim filter
if $local_part is "example"
then
    unseen save /tmp/example_dir
endif

Nothing gets written in logs, nothing gets saved (normal delivery occurs of course).

When I change $local_part in system filter file to root and test it like so:

% exim  -bF /etc/exim4/system_filter -d-all+filter -f [email protected]  <tfpmet 
Exim version 4.89 uid=0 gid=0 pid=1261 D=200
...
Return-path taken from "Return-path:" header line
Return-path = [email protected]
Sender      = [email protected]
Recipient   = [email protected]
Testing Exim filter file "/etc/exim4/system_filter"

Condition is true: $local_part is root
Unseen save message to: /tmp/example_dir
Filtering did not set up a significant delivery.
Normal delivery will occur.
  >>>>>>>>>>>>>>>> Exim pid=1261 terminating with rc=0 >>>>>>>>>>>>>>>>

It clearly says:

Condition is true: $local_part is root
Unseen save message to: /tmp/example_dir

However, nothing gets saved again.

OS: Debian 9.11 amd64.

I'd prefer to achieve this result using system filter, but any good solution would do really.