unauthorized 401 error while using httpClient in C#

I am using httpClient to call a WebApi protected with Azure AAD.
I call the MSAL library to fetch the token and set up the headers like below.

        private async void SetHeaders(HttpClient HttpClientObj)
        {
            IConfidentialClientApplication app;
            app = ConfidentialClientApplicationBuilder.Create(_configurationSettings.TestAppClientId)
                                    .WithClientSecret(_configurationSettings.TestAppClientSecret)
                                    .WithAuthority(new Uri(_configurationSettings.TestAppInstance + _configurationSettings.TestAppTenantId))
                                    .Build();

            AuthenticationResult result = null;

            IEnumerable<string> scopes = new List<string>() { _configurationSettings.TestAppAuthScope };
            try
            {
                result = await app.AcquireTokenForClient(scopes)
                                  .ExecuteAsync();
            }
            catch (MsalServiceException ex)
            {
                _logger.LogError("Exception Message: {ex}");
            }

            var defaultRequestHeaders = HttpClientObj.DefaultRequestHeaders;
            defaultRequestHeaders.Clear();

            if (defaultRequestHeaders.Accept == null || !defaultRequestHeaders.Accept.Any(m => m.MediaType == "application/json"))
            {
                HttpClientObj.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
            }

            try
            {
                defaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Bearer", result.AccessToken);
            }
            catch (Exception ex)
            {
                _logger.LogError("Exception Message: {ex}");
            }
        }

        private async Task<HttpResponseMessage> CallTestAppAPIToGetDataAsync(string testAppUri)
        {
                HttpResponseMessage response = null;

                HttpClient testHttpClientObj = new HttpClient();
                SetHeaders(testHttpClientObj);

                response = await testHttpClientObj.GetAsync(requestUri: gridstateUri);

                return response;
        }

I added one breakpoint in line where the httpClientObj calls the API and I can see the Authorization token with the correct value.
When I try to hit the same uri with in postman with the same token it works fine, but here it gives 401 Unauthorized.

                response = await testHttpClientObj.GetAsync(requestUri: gridstateUri);

You have declared SetHeaders method as async and using await inside, so try fixing your code by changing declaration to async Task

private async Task SetHeaders(HttpClient HttpClientObj)
{
    ...
}

and then awaiting the result of this method by await SetHeaders(testHttpClientObj) otherwise the program continues without waiting for the task to complete