Set_keepalive alway return "closed while sending to client"

We plan to use Lua code blocks to call external services to record some logs in Nginx services, But the pressure test found poor performance and high latency.

I found that http.set_keepalive has error response and keepalive is not taking effect. I have pasted some code as follow, could you please help me analyze it?

Error response:
*1375 [lua] content_by_lua(nginx.conf:97):27: failed to set keepalive: closed while sending to client, client: 192.168.71.11, server: _, request: "POST /v2/models/aiml_model/infer HTTP/1.1", host: "aiplatform.dev51.cbf.dev.paypalinc.com-shadow"

`

   location / {
        mirror /mirror1;
        resolver 192.168.36.10 valid=10s;
        proxy_pass  http://backends;
        proxy_method $request_method;
        mirror_request_body on;
        proxy_pass_request_body on;

        proxy_set_header Host $host:$server_port;
        proxy_set_header X-Real-IP $remote_addr;
        proxy_set_header X-Real-PORT $remote_port;
        proxy_set_header X-Forwarded-Proto $scheme;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;

        content_by_lua_block {
            local http = require "resty.http"
            local ngx_log = ngx.log
            local ngx_ERR = ngx.ERR
            local httpc = http.new()
            httpc:set_timeout(5000)

            local url = "https://aiplatform.dev52.cbf.dev.paypalinc.com/bizlogging/insert?model_id=model_id"
            local body = '{"metadata_schema": ["request_body"],"variable_schema": ["response_body"]}'
            local headers = {
                ["Content-Type"] = "application/json",
                ["accept"] = "*/*",
            }

           local res, err = httpc:request_uri(url, {
               method = "POST",
               body = body,
               headers = headers,
               ssl_verify = false,
               timeout = 1000,
           })

           ngx_log(ngx_ERR, "success to request: ", res.status)

            local ok, err1 = httpc:set_keepalive(60000, 100)
            if not ok then
                ngx_log(ngx_ERR, "failed to set keepalive: ", err1)
            end
        }
    }`

Start by reproducing the shown OpenResty configuration and tracing the httpc:request_uri call through httpc:set_keepalive. Check the reported “closed while sending to client” response and determine the conditions under which keepalive fails; done means a confirmed cause and a project-appropriate fix or documented resolution.