Socket.IO to().emit() works locally but not on EC2 behind an ALB (WSS configured)

Question

I’m using NestJS with Socket.IO and RabbitMQ. Locally, everything works as expected: when the client emits a send_message event, the server processes it and then sends a receive_message event to the correct room, which the client receives. However, once I deploy this to an AWS EC2 instance behind an Application Load Balancer (ALB) with WSS configured, the client never receives the receive_message event.

Below is a simplified version of my code.

// socketio.gateway.ts
@WebSocketGateway({
  namespace: "",
  cors: { origin: "*" },
  transports: ["websocket"]
})
export class SocketIoGateway implements OnGatewayInit, OnGatewayConnection, OnGatewayDisconnect {
  @WebSocketServer()
  server: Server;

  // ... other methods ...
}

// my-socket-service.service.ts
@Injectable()
export class MySocketService implements OnModuleInit {
  constructor(
    private readonly eventsGateway: SocketIoGateway,
    private readonly amqpConnection: AmqpConnection,
    // ...
  ) {}

  async onModuleInit() {
    // Register Socket.IO event
    this.eventsGateway.registerEvent("send_message", this.handleSendMessage.bind(this));
  }

  private async handleSendMessage(client: Socket, data: FileMessageInput) {
    // Process incoming data and publish via RabbitMQ
    await this.publishMessage(plainToInstance(DataDTO, data));
  }

  async publishMessage(dataDTO: DataDTO) {
    try {
      await this.amqpConnection.publish("queue", "message.new", dataDTO);
    } catch (error) {
      // ...
      throw error;
    }
  }

  @RabbitSubscribe({
    exchange: "queue",
    routingKey: "message.new",
    queue: "my_queue",
  })
  async handleMessage(data: DataDTO, amqpMsg: ConsumeMessage) {
    // Once the message is processed, emit to the correct room
    try {
      const resData = await this.processMessage(data);
      await this.eventsGateway.server
        .to(`room-${resData.roomId}`)
        .emit("receive_message", resData); // <-- This works locally, but not on EC2
      // ...
    } catch (error) {
      // ...
    }
  }
}

What I’ve tried:

1. Local environment (Mac/Windows) → Works perfectly. Client emits send_message, server eventually sends receive_message.
2. Production environment (Amazon Linux EC2)
   • Deployed behind an ALB with WSS configured.
   • I’ve confirmed that the connection is upgrading to WebSocket successfully.
3. RedisAdapter: I’ve already configured a Redis adapter for Socket.IO to handle scalability. No issues or errors are logged, but the receive_message events simply do not arrive at the client.

Symptoms and Debugging:

• The send_message event is definitely hitting the server, as I can see logs for it being processed.
• The RabbitMQ message is published and consumed by handleMessage(...) without errors.
• The final this.eventsGateway.server.to(...).emit(...) call doesn’t cause any errors on the server side, but the client sees nothing.
• ALB’s health checks are OK. The WebSocket connection is established, just no events arrive for receive_message.

Question:

• What could cause to(...).emit(...) events to work locally but fail to reach the client in AWS with an ALB and WSS setup?
• Are there any special configurations or known issues with NestJS Socket.IO gateways, the Redis adapter, or AWS ALB that I’m missing?

Any guidance or suggestions would be greatly appreciated. Thanks in advance!