Google Recaptcha V3 Invalid-Input-Response

I have the following .Net framework code:

    public static ReCaptchaResponse VerifyCaptcha(string secret, string response)
   {
       ReCaptchaResponse res = new ReCaptchaResponse();
       res.success = false;

       if (HttpContext.Current.Request != null)
       {
           using (System.Net.Http.HttpClient hc = new System.Net.Http.HttpClient())
           {
               var values = new Dictionary<string,
                   string> {
                   {
                       "secret",
                       secret
                   },
                   {
                       "response",
                       response
                   }
               };
               var content = new System.Net.Http.FormUrlEncodedContent(values);
               var Response = hc.PostAsync("https://www.google.com/recaptcha/api/siteverify", content).Result;
               var responseString = Response.Content.ReadAsStringAsync().Result;
               if (!string.IsNullOrWhiteSpace(responseString))
               {
                   res = JsonConvert.DeserializeObject<ReCaptchaResponse>(responseString);
                   return res;
               }
               else
                   return res;
           }
       }
       else
           return res;
   }

I tried to move it to .Net core, so now I have:

 public ReCaptchaResponse VerifyReCaptcha(string secret, string response)
    {
        if (string.IsNullOrWhiteSpace(secret) || string.IsNullOrWhiteSpace(response))
        {
            throw new ArgumentException("Secret and response are required.");
        }

        var reCaptchaResponse = new ReCaptchaResponse { Success = false };

        try
        {
            var values = new Dictionary<string, string>
{
    { "secret", secret },
    { "response", response }
};

            var content = new FormUrlEncodedContent(values);

            // Ensure the Content-Type is set properly
            content.Headers.ContentType = new System.Net.Http.Headers.MediaTypeHeaderValue("application/x-www-form-urlencoded");

            // Make the HTTP POST call synchronously
            using (var httpClient = new HttpClient())
            {
var httpResponse = httpClient.PostAsync("https://www.google.com/recaptcha/api/siteverify", content).Result;

                if (httpResponse.IsSuccessStatusCode)
                {
                    var responseString = httpResponse.Content.ReadAsStringAsync().Result;
                    reCaptchaResponse = JsonConvert.DeserializeObject<ReCaptchaResponse>(responseString);
                }
                else
                {
                    Console.WriteLine($"Google reCAPTCHA returned {httpResponse.StatusCode}: {httpResponse.ReasonPhrase}");
                }
            }
        }
        catch (Exception ex)
        {
            Console.WriteLine($"Error during reCAPTCHA verification: {ex.Message}");
        }

        return reCaptchaResponse;

    }

The first function works normally, and the captcha is being validated, while the other returns "invalid input response.".

I double checked the secret key and the g-captcha response, and I am sure they are correct.

What could be the issue?

I am thinking of something built-in in .NET core that altering the request?