Prevent nginx from rewriting a port in location header

so I run a Laravel application in an nginx container, to which I give access to through another nginx acting as a reverse proxy. The container exposes a port 8123/tcp for accessing the service. The problem is that for some reason nginx adds a port to the address at some point, so let's say I access https://app.example.com, then the container redirects to http://app.example.com/docs and then redirects to http://app.example.com:8123/docs/. Here is the configuration I use for the reverse proxy:

server {
        listen 443 ssl http2;
        listen [::]:443 ssl http2;
        server_name app.example.com;
        access_log /var/log/nginx/app.example.com-access.log;
        error_log /var/log/nginx/app.example.com-errors.log;
        add_header Referrer-Policy same-origin;
        add_header X-Content-Type-Options nosniff;
        add_header X-XSS-Protection "1; mode=block";
        add_header X-Frame-Options "SAMEORIGIN";
        add_header Strict-Transport-Security "max-age=31536000; includeSubDomains; preload";

        location / {
                proxy_redirect off;
                port_in_redirect off;
                proxy_pass http://127.0.0.1:8123;
                proxy_http_version 1.1;
                proxy_set_header Host $http_host;
                proxy_set_header Location $http_host;
                proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
                proxy_set_header X-Forwarded-Host $http_host;
                proxy_set_header X-Real-IP $remote_addr;
        }
    ssl_certificate /etc/letsencrypt/live/example.com/fullchain.pem;
    ssl_certificate_key /etc/letsencrypt/live/example.com/privkey.pem;
    ssl_dhparam /etc/letsencrypt/dhparams_4096.pem;
}
server {
        listen 80;
        listen [::]:80;
        server_name app.example.com;
        return 302 https://app.example.com$request_uri;
}

This one is for the container with nginx inside:

worker_processes 1;
error_log stderr debug;
pid /run/nginx.pid;

user  www-data; #user must be the same as the one running php

events {
    worker_connections 1024;
}

http {
    include mime.types;
    default_type application/octet-stream;
    sendfile on;
    tcp_nopush on;
    tcp_nodelay on;
    fastcgi_keep_conn on;
    fastcgi_buffering on;
    fastcgi_buffer_size 512k;
    fastcgi_buffers 128 256k;

    # Tune nginx keepalives to work with the GCP HTTP(S) Load Balancer:
    keepalive_timeout 65;

    # Define custom log format to include reponse times
    log_format main_timed '$remote_addr - $remote_user [$time_local] "$request" '
                          '$status $body_bytes_sent "$http_referer" '
                          '"$http_user_agent" "$http_x_forwarded_for" '
                          '$request_time $upstream_response_time $pipe $upstream_cache_status';

    access_log /dev/stdout main_timed;
    error_log /dev/stderr debug;

    # Write temporary files to /tmp so they can be created as a non-privileged user
    client_body_temp_path /tmp/client_temp;
    proxy_temp_path /tmp/proxy_temp_path;
    fastcgi_temp_path /tmp/fastcgi_temp;
    uwsgi_temp_path /tmp/uwsgi_temp;
    scgi_temp_path /tmp/scgi_temp;



    # Default server definition
    server {
        listen 8123;
        server_name _;
        access_log /dev/stdout main_timed;
        error_log /dev/stderr debug;
        index index.php index.html;

        add_header X-Frame-Options "SAMEORIGIN";
        add_header X-XSS-Protection "1; mode=block";
        add_header X-Content-Type-Options "nosniff";

        # Defined in Cloud run
        client_max_body_size 300M;

        root /var/www/app/public;

        location / { try_files $uri $uri/ try_files $uri $uri/ /index.php$is_args$args; }

        location ~ .php$ {
                include /etc/nginx/fastcgi_params;
                fastcgi_pass unix:/tmp/php-fpm.sock;

                fastcgi_index index.php;
                fastcgi_read_timeout 300;
                fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
        }

        location ~ /\. {
                deny all;
        }

        location = /favicon.ico {
            access_log     off;
            log_not_found  off;
        }
        location ~*  \.(woff|woff2|svg|jpg|jpeg|png|gif|ico)$ {
            expires 1d;
        }
        location ~* \.(js|css)$ {
            expires 20m;
        }
        location ^~ /storage/app/docs {
                    deny all;
        }
        # Allow fpm ping and status from localhost
        # location ~ ^/(fpm-status|fpm-ping)$ {
        #     access_log off;
        #     allow 127.0.0.1;
        #     deny all;
        #     #allow all;
        #     fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
        #     include fastcgi_params;
        #     fastcgi_pass unix:/tmp/php-fpm.sock;
        # }
    }

    gzip on;
    gzip_proxied any;
    gzip_types text/plain application/xml text/css text/js text/xml application/x-javascript text/javascript application/json application/xml+rss;
    gzip_vary on;
    gzip_disable "msie6";

    # Include other server configs
    include /etc/nginx/conf.d/*.conf;

EDIT 1: I haven't noticed that I'm including other config files, so here are they:

fastcgi_params

fastcgi_param  QUERY_STRING       $query_string;
fastcgi_param  REQUEST_METHOD     $request_method;
fastcgi_param  CONTENT_TYPE       $content_type;
fastcgi_param  CONTENT_LENGTH     $content_length;

fastcgi_param  SCRIPT_NAME        $fastcgi_script_name;
fastcgi_param  REQUEST_URI        $request_uri;
fastcgi_param  DOCUMENT_URI       $document_uri;
fastcgi_param  DOCUMENT_ROOT      $document_root;
fastcgi_param  SERVER_PROTOCOL    $server_protocol;
fastcgi_param  REQUEST_SCHEME     $scheme;
fastcgi_param  HTTPS              $https if_not_empty;

fastcgi_param  GATEWAY_INTERFACE  CGI/1.1;
fastcgi_param  SERVER_SOFTWARE    nginx/$nginx_version;

fastcgi_param  REMOTE_ADDR        $remote_addr;
fastcgi_param  REMOTE_PORT        $remote_port;
fastcgi_param  SERVER_ADDR        $server_addr;
fastcgi_param  SERVER_PORT        $server_port;
fastcgi_param  SERVER_NAME        $server_name;

# PHP only, required if PHP was built with --enable-force-cgi-redirect
fastcgi_param  REDIRECT_STATUS    200;

/etc/nginx/conf.d folder is empty.

What do I do wrong?