HttpClient headers repro

client.cs

using System;
using System.Linq;
using System.Net.Http;
using System.Reflection;
using System.Runtime.InteropServices;
using System.Threading.Tasks;

namespace HttpClientTest
{
    class Program
    {
        public static void Main(string[] args)
        {
            DoWorkAsync().GetAwaiter().GetResult();
        }

        public static async Task DoWorkAsync()
        {
            using (var httpClientHandler = new HttpClientHandler())
            {
                using (var client = new HttpClient(httpClientHandler))
                {
                    Console.WriteLine(RuntimeInformation.FrameworkDescription);
                    Console.WriteLine(Assembly.GetAssembly(typeof(HttpClient)));
                    var response = (await client.GetAsync("http://localhost:5000/api/values"));
                    response.Headers.TryGetValues(_serverKey, out var serverName);
                    
                    // This always works
                    Console.WriteLine($"Headers.TryGetValues({_serverKey}): " + serverName?.First());
                    
                    // Using `TryGetValues("Authorization")` returns empty string with .NET Core 2.1, but not with .NET Framework 4.7
                    response.Headers.TryGetValues(_authorizationKey, out var authorization);
                    Console.WriteLine($"Headers.TryGetValues({_authorizationKey}): {authorization?.First()}");
                    
                    // Iterating over Headers works as expected, though
                    Console.WriteLine($"Headers.First(x => x.Key == {_authorizationKey}): {response.Headers.First(x => x.Key == _authorizationKey).Value.FirstOrDefault()}");
                }
            }
        }

        const string _serverKey = "Server";
        const string _authorizationKey = "Authorization";
    }
}

server.cs

// Create a sample API with dotnet new webapi, then return an Authorization header

public ActionResult<IEnumerable<string>> Get()
{
    Response.Headers.Add("Authorization", "Bearer dpmtest");
    return Ok("");
}

With dotnet version 2.1.300-rc1-008673:

NET Core 4.6.26426.02
System.Net.Http, Version=4.2.1.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a
Headers.GetValues(Server): Kestrel
Headers.GetValues(Authorization): 
Headers.First(x => x.Key == Authorization): Bearer dpmtest

With a .NET 4.7 app referencing a .NET Standard 2.0 library

System.Net.Http, Version=4.2.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a
Headers.GetValues(Server): Kestrel
Headers.GetValues(Authorization): Bearer dpmtest
Headers.First(x => x.Key == Authorization): Bearer dpmtest

With just a .NET 4.7 app:

System.Net.Http, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a
Headers.GetValues(Server): Kestrel
Headers.GetValues(Authorization): Bearer dpmtest
Headers.First(x => x.Key == Authorization): Bearer dpmtest