Nginx Blocking task with long processing time

Question

I'm trying to perform horizontal scaling of long processing task with docker compose. To simulate that task, I used the following code in python:

@app.get("/test")
def test():
    start = time.time()
    print("Start time", time.asctime(time.localtime(start)))
    time.sleep(5)
    end = time.time()
    print("End time", time.asctime(time.localtime(end)))
    return {"start":start, "end":end}

In docker compose I created 2 containers for the same code above. Mainly back1 and back2

version: '3.8'
services:
  back1:
    build:
      context: ./backend
      dockerfile: DockerFile
    volumes:
      - ./backend:/usr/code
    command: python main.py
    ports: 
      - "8001"
  back2:
    build:
      context: ./backend
      dockerfile: DockerFile
    volumes:
      - ./backend:/usr/code
    command: python main.py
    ports: 
      - "8001"

  nginx:
    build:
      context: ./backend/config
      dockerfile: DockerFile
    ports:
      - "80:80"

And this is my nginx conf file:

upstream backend {
    server back1:8001;
    server back2:8001;

    keepalive 32;
}

server {
    listen 80;
    server_name api.localhost;
    location / {
        proxy_pass http://backend;
    }
}

I tried to simultaneously trigger the API http://api.localhost/test. However it seems that the second request was activated after the 1st request is completed. Even though it was processed by different instance of container.

Is nginx actually blocking?? I know that we can use queueing (eg: rabbitMQ, etc), but I thought that spinning up 2 docker container and using nginx as load balancer would achieve horizontal scalling. Am I wrong?