Why is the log file of UdpProxy of Envoy always empty

I'm using Envoy as my gateway of my backend micro-services, some of which provides some UDP services. So I need to configure my Envoy to route the UDP requests to the services.

Here is my config file of Envoy:

static_resources:
  listeners:
  - name: listener_udp_test
    reuse_port: true
    address:
      socket_address:
        protocol: UDP
        address: 0.0.0.0
        port_value: 22221
    access_log:
    - name: envoy.access_loggers.file
      typed_config:
        "@type": type.googleapis.com/envoy.extensions.access_loggers.file.v3.FileAccessLog
        path: "/home/administrator/envoy/udp_test.log"  # always empty!!!
    udp_listener_config:
      downstream_socket_config:
        max_rx_datagram_size: 9000
    listener_filters:
    - name: envoy.filters.udp_listener.udp_proxy
      typed_config:
        '@type': type.googleapis.com/envoy.extensions.filters.udp.udp_proxy.v3.UdpProxyConfig
        stat_prefix: example_ingress_udp
        cluster: cluster_udp_test
        upstream_socket_config:
          max_rx_datagram_size: 9000
  clusters:
  - name: cluster_udp_test
    connect_timeout: 30s
    type: LOGICAL_DNS
    dns_lookup_family: V4_ONLY
    load_assignment:
      cluster_name: cluster_udp_test
      endpoints:
      - lb_endpoints:
        - endpoint:
            address:
              socket_address:
                address: 172.1.2.3
                port_value: 22222

As you see, the gateway is listening the port 22221 and routing requests to 172.1.2.3:22222.

Let's say the IP of the gateway is 172.1.2.2 and I have a client 172.1.2.1.

I execute nc -l -u 172.1.2.3 22222 on the server 172.1.2.3 and I execute nc -u 172.1.2.2 22221 on the client 172.1.2.1.

Now, I can send messages from 172.1.2.1 to 172.1.2.3 as exptected. Everything works fine.

172.1.2.1 <---> 172.1.2.2(gateway) <---> 172.1.2.3

But, as you see I have configured a log file named udp_test.log. But after I running the gateway, this file does appear but it is always empty no matter how I send messages. As my understanding, needn't it store some log info about sending and receiving messages? If not, which kinds of info would be stored in this log file?