Nginx proxy incorrect fastcgi_script_name for static files

server {
    listen loc.app:80;    

    root /app/frontend/web;

    index index.php;

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

    location ~ ^/admin {
        proxy_pass http://127.0.0.1:81;     
    }

    location ~* .php$ {    
        #php conf
    }                    
}

server {
    listen 127.0.0.1:81;

    root /app/backend/web;

    index index.php;

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

    location ~ .(js|css)$ {    
        echo "$document_root"; # /app/backend/web;

        echo "$fastcgi_script_name"; # /admin/assets/569a8b41/css/bootstrap.css         
        ############################## ISSUE ##############################
        # $fastcgi_script_name must be /assets/569a8b41/css/bootstrap.css #
        ###################################################################
    }

    location ~* .php$ {    
        #php conf
    }
}

http://loc.app/admin - ok
http://loc.app/admin/style.css - returns 404

How to force nginx to correctly handle static files ?