Empty message parse using ArduinoJSON from aedes MQTT server

I'm using aedes as my custom MQTT broker and PubSubClient at the ESP32 side, together with the ArduinoJSON package.

My server test code:

import Aedes from "aedes";
import net from "net";

let port = process.env.MQTT_PORT || 1883; // MQTT Port

const id = "MY_SERVER";

const aedes = new Aedes({
    id: id,
});

aedes.on("client", async (client) => {
    console.log("Client conntected...");
});

aedes.on("subscribe", async (subscriptions, client) => {

    let topic = "cmd/" + client.id;

    let msg = {
        topic: topic,
        type: "test",
        param: "p1"
    };

    console.log("Sending command...")
    console.log(msg);

    this.aedes.publish(msg);

});

let server = net.createServer(aedes.handle, options);

server.listen(port, () => {
    console.log(`Find the server at: mqtt://localhost:${port}/`); 
});

When running I'm getting:

Client connected...
Sending command...
{ topic: 'cmd/4CA9A712B42A', type: 'test', param: 'p1' }

At the client (ESP32) side I have my PubSubClient callback as follows:

void callback(char *topic, byte *payload, unsigned int length)
{
    Serial.println("Message received.....");

    StaticJsonDocument<256> doc;
    deserializeJson(doc, payload, length);

    char *command = doc["type"];
    char *param = doc["param"];

    Serial.println(command);
    Serial.println(param);
}

The callback is being called property after subscription - but I'm not being able to parse the payload:

Message received.....
<empty line>
<empty line>
    

How can I fix the server or client to allow then to send/parse correctly the payload.