Logstash Json filter behaving unexpectedly for lar

Question

This is my logstash config:

input {
    redis { host => "127.0.0.1" data_type => "list" key => "logstash" }
}

filter {
    json {
        source => 'appData'
    }
}

output {
    elasticsearch { host => '127.0.0.1' }
}

Things work fine is appData is a simple JSON, but the elasticsearch starts giving parsing errors when large nested JSONs are passed into appData.

Is there an alternative to the JSON filter ? or is there somewhere I can see the debug logs to pin point the problem.