Logstash-codec-json stopped working

I used logstash-codec-json plugin for a POC deployment on AWS, the related configuration is as below.

input {
    udp {
        port => "9999"
        type => "mylog"
        codec => json
        buffer_size => 65536
    }
}

output {
    amazon_es {
        hosts => ["dnsname.us-east-1.es.amazonaws.com"]
        index => "mylog-%{+YYYY.MM.dd}"
      }
}

It worked for a while, then stopped working w/o throwing any error.

Tcpdump shows the packet reach the udp port as configured above in proper json format.

I searched the issues on github, didn't find any clue.

The only similar issue reported and I could find online was

but it didn't have any valid solution.

I need some help to troubleshot this issue.

The version of the plugin is logstash-codec-json (3.0.5), and logstash version is 6.8.8.

Thanks.

Paul