HttpClient always do Basic Authentication check after provide an authorization header? - asp.net-web-api

Question

The web api control, UserControl, has two methods, RetrieveUserID which needs Basic Authorization check
[HttpGet]
[Route("RetrieveUserID/{strUsername}")]
[Authorize]
public string RetrieveUserID(string strUsername)
{
//retrieve userID and return it
......
return strUserID;
}
Another method, FailAuthenticationReason, is used if fail to retrieve userID, it returns the detail fail info such as wrong username, wrong password, account is locked out, etc. which doesn't need to do any authentication check
[HttpGet]
[Route("FailAuthenticationReason/{strUsername}")]
public string FailAuthenticationReason(string strUsername)
{
//retrieve detail failed reason
......
return strFailedReason;
}//End of
It works well when I use a browser to check them. But when I use it in my app, after I provide the authorization header and fail to retrieve userID because of incorrect username and/or password, it also do the authorization check when it call FailAuthenticationReason
var authData = string.Format("{0}:{1}", entUsername.Text,entPassword.Text);
var authHeaderValue = Convert.ToBase64String(Encoding.UTF8.GetBytes(authData));
App.httpClient.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Basic", authHeaderValue);
var uri = new Uri(string.Format(App.strWebAPIURI + "/RetrieveUserID/{0}", entUsername.Text));
try
{
var response = await App.httpClient.GetAsync(uri);
if (response.IsSuccessStatusCode)
{
......
}
else
{
//Fail to pass authorization
uri = new Uri(string.Format(App.strWebAPIURI + "/FailAuthenticationReason/{0}", entUsername.Text));
response = await App.httpClient.GetAsync(uri);
......
}
How can the program call FailAuthenticationReason without the authorization check?