AWS IoT Core / mqtt.js - After Successful Topic Subscription, Still Not Able To Run a Callback On a Message

I'm trying to use the mqtt.js (https://github.com/mqttjs/MQTT.js) library in order to interact with the AWS IoT Core endpoint with the following code:

const mqtt = require("mqtt");

const IOT_ENDPOINT = "wss://xxxxxx-ats.iot.eu-central-1.amazonaws.com";
const client = mqtt.connect(`${IOT_ENDPOINT}/mqtt?x-amz-customauthorizer-name=Authorizer`);

client.on("connect", () => {
    client.subscribe("xyz", err => {
        if (!err) {
            // Works, I can see the message in AWS Console / MQTT test client.
            const dt = new Date().toISOString();
            client.publish("xyz", JSON.stringify({ message: "CURRENT DT:" + dt }));
        }
    });
});

// The callback never gets executed, nothing is printed in the console.
client.on("message", (topic, message) => {
    console.log("Message received.");
    client.end();
});

What's interesting is that I can see the published message in the xyz topic in the AWS Console / MQTT test client. Great!

What I can also see is that there are no messages being printed in my terminal, despite the fact that the client.subscribe call did not throw any errors (in other words, topic subscription was successful), and I am able to publish messages on the same topic from the AWS Console / MQTT test client.

I've tried multiple things, changing params when connecting, trying different variations of the code, no luck 😩.

My next idea on how to debug is to try enabling IOT CW logs and see if anything is logged in there maybe.

Until then, my question is: anybody have any suggestions on what else to check why nothing is being printed in my terminal?

Thank you!🍻

UPDATE: Sharing my AWS Lambda authorizer code, maybe it's something related to permissions? 🤔

module.exports.handler = async (event, _, callback) => {
    return ({
        isAuthenticated: true,
        principalId: 'Unauthenticated',
        policyDocuments: [
            {
                "Version": "2012-10-17",
                "Statement": [
              
                    {
                        "Effect": "Allow",
                        "Action": "iot:Connect",
                        "Resource": "arn:aws:iot:*:*:client/*"
                    },
                    {
                        "Effect": "Allow",
                        "Action": "iot:Subscribe",
                        "Resource": [
                             // Added '*' for testing purposes.
                            '*',
          "arn:aws:iot:*:*:client/*",
                            "arn:aws:iot:*:*:topicfilter/server"
                            
                            
                        ]
                    },
                    {
                        "Effect": "Allow",
                        "Action": "iot:Publish",
                        "Resource": [
                            "arn:aws:iot:*:*:client/*",
                            "arn:aws:iot:*:*:topic/xyz"
                        ]
                    }
                ]
            }
        ],
        disconnectAfterInSeconds: 3600,
        refreshAfterInSeconds: 300
    });
};