_dateparsefailure, can't debug further

Elastic Stack Logstash
5

I tried using the mutate+convert strategy but still got _dateparsefailure:

filter {
    mutate {
        copy => {"@timestamp" => "timestamp" }
        convert => { "timestamp" => "string" }    
    }

    date {
        match => ["timestamp", "ISO8601"]
        locale => "en-US"
        timezone => "America/Sao_Paulo"
        target => "timestamp"
    }
}

but why would you want to that? It is already parsed!

Basically, I'm sending this output to a Big Data Platform that is expecting to receiving this input:

{"@timestamp":"2019-08-26T23:48:31.931Z","timestamp":"2019-08-26T20:48:30.347-03:00"}

So I'm trying to convert the bellow, into the above:

{"@timestamp":"2019-08-26T23:48:31.931Z","timestamp":1567637837587}