Codeigniter in subfolder with nginx

multi-ci.conf

server {
    listen 80;
    listen [::]:80;

    server_name _;
    root /var/www/{root_folder};
    index index.php index.html;

    location / {
        access_log {access_log_file{;
        error_log {errors_log_file};

        try_files $uri $uri/ /index.php;
    }
      
    location ~ \.php$ {
        try_files $uri /index.php =404;
        include fastcgi_params;
        fastcgi_index index.php;
        fastcgi_param SCRIPT_FILENAME $request_filename;
        fastcgi_split_path_info ^(.+\.php)(/.+)\$;
        fastcgi_pass {php-fpm |if php7.2 use: unix:/run/php/php7.2-fpm.sock};
        fastcgi_buffers 16 16k;
        fastcgi_buffer_size 32k;
        fastcgi_read_timeout 600;
    }

    location ~ /\.ht { deny all; }

    location ~ ^/(application|system|tests)/ { return 403; }

    location ^~ /subfolder {
        alias /var/www/dev/{sub_folder};
       
        access_log {access_log_file{;
        error_log {errors_log_file};

        #if (!-e $request_filename) { rewrite ^ /subfolder/index.php last; }
        
        try_files $uri /subfolder/index.php;

        location ~ \.php$ {
            try_files $uri /index.php =404;
            include fastcgi_params;
            fastcgi_index index.php;
            fastcgi_param SCRIPT_FILENAME $request_filename;
            fastcgi_split_path_info ^(.+\.php)(/.+)\$;
            fastcgi_pass {php-fpm |if php7.2 use: unix:/run/php/php7.2-fpm.sock};
            fastcgi_buffers 16 16k;
            fastcgi_buffer_size 32k;
            fastcgi_read_timeout 600;
        }

        location ~ /\.ht { deny all; }

        location ~ ^/(application|system|tests)/ { return 403; }
    }
    
}