Logstash config file interpolation issue

[human39 (Scott)]

I'm having an issue that I'm finally giving up on and I need to ask for some help.

Here is what I'm doing:

    ...
    geoip {
        source => "[system][auth][ssh][ip]"
        target => "[system][auth][ssh][geoip]"
    }

    memcached {
        hosts => ["ip.add.re.ss:11211"]
        get => {
            "%{[system][auth][ssh][ip]}" => "[company][ip_reputation][source]"
        }
        add_tag => [ "ip_reputation_hit" ]
    }
    ...

The idea is that I'm looking up ip's in a memcached "db" to see if they match reputation lists right before indexing, but that's not the issue: The issue is that it's sending a literal %{[system][auth][ssh][ip]} to the memcached server, confirmed with a tcpdump. I included the geoip block to show that I do have [system][auth][ssh][ip] configured.

The most confusing part is that I'm not able to recreate the issue in testing. I created the following test config:

input {
  stdin {
    codec => json
  }
}
filter {

  mutate {
    copy => ["ip", "[system][auth][ssh][ip]"]
  }

  memcached {
     hosts => ["ip.ad.dre.ss:11211"]
     get => {
        "%{[system][auth][ssh][ip]}" => "[company][ip_reputation][source]"
     }
     add_tag => "ip_reputation_hit"
  }

}
output {
  stdout {
     codec => rubydebug
  }
}

and if I test it with the input { "ip" : "1.1.1.1" }, it works as I would expect it to using the same version of logstash (6.5.4) I'm using in production.

I hope this is something simple that I'm just overlooking. Thanks for looking!