coredns: plugin/file Transfer does not seem to work

What happened: I have a 3rd party DNS hosting service that points to a CoreDNS server and does notifies to when changes are made. This works fine, but I am trying to have more CoreDNS servers that this node would notify of the changes after it receives them. From reading the docs, I feel like I configured it right, but I see no traffic going out of CoreDNS using tcpdump or any notifies on the second server’s logs

What you expected to happen: I was expecting server 1 to send a notify or transfer to server 2

How to reproduce it (as minimally and precisely as possible): Configs below

Anything else we need to know?:

Environment: 2 servers in the same subnet with an external IP on server 1

• the version of CoreDNS: 1.9.3
• Corefile: <SERVER 1>

.:53 {
    forward . <other DNS server IP> {
        except example.org
    }
}
example.org:53 {
  secondary {
    transfer from <3rd party IP>
  }
  transfer {
    to * <internal IP>
  }

  health :8080
  log all
}

SERVER 2

.:53 {
    forward . <other DNS server IP> {
        except example.org
    }
}
example.org:53 {
  secondary {
    transfer from <Server 1's IP>
  }
  health :8080
  log all
}

• logs, if applicable:

Sep 07 17:06:25 zgqfwavkndqqnrwcvvaf-sysrv.example.org [1]: Started CoreDNS DNS server.
Sep 07 17:06:25 zgqfwavkndqqnrwcvvaf-sysrv.example.org coredns[6577]: .:53
Sep 07 17:06:25 zgqfwavkndqqnrwcvvaf-sysrv.example.org coredns[6577]: example.org.:53
Sep 07 17:06:25 zgqfwavkndqqnrwcvvaf-sysrv.example.org coredns[6577]: CoreDNS-1.9.3
Sep 07 17:06:25 zgqfwavkndqqnrwcvvaf-sysrv.example.org coredns[6577]: linux/amd64, go1.18.2, 45b0a11
Sep 07 17:06:26 zgqfwavkndqqnrwcvvaf-sysrv.example.org coredns[6577]: [INFO] plugin/file: Transferred: example.org. from <3rd party IP>:53
Sep 07 17:06:35 zgqfwavkndqqnrwcvvaf-sysrv.example.org coredns[6577]: [INFO] plugin/file: Notify from <3rd party IP> for example.org.: checking transfer
Sep 07 17:06:36 zgqfwavkndqqnrwcvvaf-sysrv.example.org coredns[6577]: [INFO] plugin/file: Transferred: example.org. from 208.78.68.66:53
Sep 07 17:06:58 zgqfwavkndqqnrwcvvaf-sysrv.example.org coredns[6577]: [INFO] plugin/file: Notify from <3rd party IP>  for example.org.: checking transfer
Sep 07 17:06:59 zgqfwavkndqqnrwcvvaf-sysrv.example.org coredns[6577]: [INFO] plugin/file: Transferred: example.org. from <3rd party IP>:53

• OS (e.g: cat /etc/os-release): Centos 7
• Others: