Nginx post请求似乎正在删除我的头文件，那么我如何才能让头文件“Access-Control-Allow-Origin”保留在post请求中呢？

location /api {
        proxy_pass http://127.0.0.1:3567/;

        if ($request_method = OPTIONS) {
            add_header "Access-Control-Allow-Origin" "http://localhost:8080";
            add_header "Access-Control-Allow-Methods" "GET, POST, OPTIONS";
            add_header "Content-Type" "text/plain";
            add_header "Access-Control-Allow-Credentials" "true";
            add_header "Access-Control-Allow-Headers" "content-type,rid,fdi-version";
            add_header "Content-Length" 0;
            return 204;
        }

        add_header "Access-Control-Allow-Origin" "http://localhost:8080";
    }

现在，当request方法设置为OPTIONS时，它可以工作，但对于POST请求则不行。我希望在响应中有“Access-Control-Allow-Origin”头，但是没有