PKCE flow Error code: 500 code challenge required - xero-api

Question

I'm trying to get the PKCE example to work, but I keep hitting
Error code: 500
Error: invalid_request : code challenge required
Here's a sample url, it does include a code_challenge param generated with the example code.
https://login.xero.com/identity/connect/authorize
?client_id=XXX
&response_type=code
&scope=openid%20profile%20email%20offline_access%20files%20accounting.transactions%20accounting.contacts&redirect_uri=https%3A%2F%2Flocalhost%3A5001%2F
&code_challenge=tj6n3SLd6FZ8g6jjSJYvfC--4r2PHGnpbSGTwIreNqQ
&code_challenge_method=S256
The registered app is a PKCE flow, kind of out of options what it could be.
Here's the code I use, the only changes are the last 2 lines where I launch the browser a I'm connecting from a desktop app. Tried pasting the generated url into the browser directly but that also didn't work.
XeroConfiguration xconfig = new XeroConfiguration();
xconfig.ClientId = "XXX";
xconfig.CallbackUri = new Uri("https://localhost:5001"); //default for standard webapi template
xconfig.Scope = "openid profile email offline_access files accounting.transactions accounting.contacts";
//xconfig.State = "YOUR_STATE"
var client = new XeroClient(xconfig);
// generate a random codeVerifier
var validChars = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz-._~";
Random random = new Random();
int charsLength = random.Next(43, 128);
char[] randomChars = new char[charsLength];
for (int i = 0; i < charsLength; i++) {
randomChars[i] = validChars[random.Next(0, validChars.Length)];
}
string codeVerifier = new String(randomChars);
var uri = client.BuildLoginUriPkce(codeVerifier);
Clipboard.SetText(uri);
System.Diagnostics.Process.Start("explorer.exe", $"\"{uri}\"");