Can something in my Nginx config imply why my backend is not sending the 'Access-Control-Allow-Origin' header in POST request?

*Edit 1: The error seem to be only with POST requests

I have a frontend website on localhost. There is a registration page on localhost/register

The website calls a backend function to register a user at localhost:8080/api/register

I use Axios to POST the username and password. The browser sends two requests: OPTIONS pre-flight request, and then the POST request.

The user is created successfully, however the browser throws an error for the POST request:

Reason: CORS header ‘Access-Control-Allow-Origin’ missing

And indeed it's missing in the response to the POST. Assuming my backend cors file is configured properly, could the issue be from the combination of my Docker + Nginx setup that blocks it or proxy the headers to a wrong place?

This is my nginx config:

server {
    listen 8080;
    index index.php index.html;    
    error_log /var/log/nginx/error.log;
    access_log /var/log/nginx/access.log;
    root /var/www/html/public;
    
    location / {
        try_files $uri $uri/ /index.php?$query_string;
    }
    
    location ~ \.php$ {        
        try_files $uri = 404;
        fastcgi_split_path_info ^(.+\.php)(/.+)$;
        fastcgi_pass php:9000;
        fastcgi_index index.php;
        include fastcgi_params;
        fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
        fastcgi_param PATH_INFO $fastcgi_path_info;
    }
    
}

  server {    
    listen       80;       
      location / {      
      proxy_pass      http://node:3000;
      
    }
  }

and this is my docker-compose.yml:

networks:
    mynetwork:
        driver: bridge

services:
    nginx:
        image: nginx:stable-alpine
        container_name: nginx
        ports:
            - "8080:8080"
            - "80:80"            
        volumes:            
            - ./php:/var/www/html 
            - ./nginx/default.conf:/etc/nginx/conf.d/default.conf
            
        depends_on:
            - php
            - node
        networks:
            - mynetwork
    
    php:        
        build:
            context: ./php
            dockerfile: Dockerfile
        container_name: php
        user: "1000:1000"
        volumes:
            - ./php:/var/www/html
        ports:
            - "9000:9000"
        networks:
            - mynetwork

    node:
        build:
            context: ./react
            dockerfile: Dockerfile
        container_name: next        
        volumes:
            - ./react:/var/www/html                
        ports:
            - "3000:3000"       

        networks:
            - mynetwork


           

**Edit 2:

The backend is Laravel and it has a CORS middleware that is supposed to take care of it. And in fact it does seem to be working because GET and OPTIONS request pass without error, only the POST request throws this error.

This is the CORS config file (cors.php) in Laravel:

'paths' => ['api/*', 'sanctum/csrf-cookie'],

'allowed_methods' => ['*'],

'allowed_origins' => ['http://localhost'],

'allowed_origins_patterns' => ['*'],

'allowed_headers' => ['*'],

'exposed_headers' => [],

'max_age' => 0,

'supports_credentials' => true