Getting intermittent 403 while making HttpPost call using Client Certificate Authentication in .net6

Question

I am trying to make HttpPost call to a web api which is protected via certificate. I am using named HttpClient and creating clients using HttpClientFactory.

services.AddHttpClient(clientName, (c) =>
                {
                    c.BaseAddress = baseAddress;
                }).ConfigurePrimaryHttpMessageHandler((sp) =>
                    {
                      // use sp and get cert
                      return new HttpClientHandler
                            {
                                ClientCertificateOptions = ClientCertificateOption.Manual,
                                SslProtocols = SslProtocols.Tls12,
                                ClientCertificates = { cert }
                            };
                     });

Now we fetch HttpClient instance using CreateClient call and using MultipartFormDataContent I make a Post request.

When I make this Post request sequentially I see not problem, when I make this request parallely on 5 differnent thread, Few call fails with 403 error. Making a GET call to same API on different endpoint in parallel doesn't translate in any error.

Any help on how can I troubleshoot this 403 error code when I make HttpPost call in parallel.

I looked at System.Net traces but unable to find what caused this.