authSwitchHandler not being triggered

Hi,

I've been trying to set up IAM authentication for my AWS RDS instance and can't seem to get it working.
I believe I have everything correct on the infrastructure side but I keep getting a Access denied error.
In the authSwitchHandler function I've added a console.log but it never seems to get triggered.

Is there any reason why it wouldn't happen?

I've put some example code that I'm trying to execute below.

Thanks,
Gary

const AWS = require('aws-sdk');
const mysql = require('mysql2/promise');

const signer = new AWS.RDS.Signer({
  region: process.env.REGION,
  hostname: process.env.DB_HOSTNAME,
  port: 3306,
  username: process.env.DB_USERNAME,
});
const token = signer.getAuthToken();

exports.main = async (event, context, callback) => {
  const config = {
    host: process.env.DB_HOSTNAME,
    user: process.env.DB_USERNAME,
    password: token,
    database: process.env.DB_NAME,
    ssl: 'Amazon RDS',
    authSwitchHandler: (data, cb) => {
      console.log('authSwitchHandler', data);
      cb(null, Buffer.from(`${token}\0`));
    },
  };
  console.log('Config', config);

  const connection = await mysql.createConnection(config);
  console.log('Connection', connection);

  return callback(null, {
    statusCode: 200,
    body: true,
  });
};

{
    "errorMessage": "Access denied for user 'user'@'x.x.x.x' (using password: YES)",
    "errorType": "Error",
    "stackTrace": [
        "Object.createConnection (/var/task/node_modules/mysql2/promise.js:31:31)",
        "exports.main (/var/task/src/handler.js:26:34)"
    ]
}

The reproduction is in src/handler.js, with the failure reported at mysql.createConnection in the stack trace; start by tracing how the mysql2 connection processes authSwitchHandler for this AWS RDS example. A useful outcome is a confirmed explanation, reproduction, and documented expected behavior or correction, since the issue does not name a target file or test.