Windows auth in aspnet core is unpredictable using IISExpress or HttpSys - authentication

Question

In my asp.net core web app I need to find a login name for the user making a request. This is done in the middleware using a name claim from HttpContext.
public static string GetUserName(HttpContext ctx)
{
var claim = ctx.User.FindFirst(ClaimTypes.NameIdentifier);
if (claim != null)
return claim.Value;
claim = ctx.User.FindFirst(ClaimTypes.Name);
var arr = claim?.Value.Split('\\');
return arr?[arr.Length - 1];
}
But sometimes Context.User.UserClaims is empty. It looks like this mostly happens when multiple requests come almost at the same time. For some of those requests claims are empty and for some they are not.
It never happens to me under IIS but does happen under IISExpress and HttpSys. I hope someone has an idea why it behaves this way.
Here's my launch.json
{
"iisSettings": {
"windowsAuthentication": true,
"anonymousAuthentication": true,
"iisExpress": {
"applicationUrl": "http://localhost:5000/",
"sslPort": 0
}
},
"profiles": {
"IIS Express": {
"commandName": "IISExpress",
"environmentVariables": {
"ASPNETCORE_ENVIRONMENT": "Development"
},
"use64Bit": true
},
"HttpSys": {
"commandName": "Project",
"launchBrowser": false,
"environmentVariables": {
"ASPNETCORE_ENVIRONMENT": "Development"
}
}
}
}