RestSharp: Post Fails Using HttpClient in RestSharp Client constructor

While working with an API recently I encountered unexpected behavior when using an existing HttpClient in the RestClient constructor.

If I use the HttpClient in the constructor all my posts return with a bad request response (code 400), however creating the RestClient with the same base url as used in the HttpClient works.

To Reproduce

TestClass:

public class Connection {
    RestClient client;

    public Connection(bool wantFail)
    {
        string     baseUrl = @"https://travellermap.com/api/";
        HttpClient http    = new HttpClient();
        http.BaseAddress = new Uri(baseUrl);

        if (wantFail) {
            client = new RestClient(http);
        }
        else {
            client = new RestClient(baseUrl);
        }
    }

    public byte[] CreateSectorMapTest() {
        byte[] result;

       
        var request = new RestRequest("poster?subsector=A", Method.Post);
        request.AddHeader("Content-Type", "text/plain");
        request.AddHeader("Accept", "image/svg+xml");

        var body = @"#--------1---------2---------3---------4---------5-------
#PlanetName   Loc. UPP Code   B   Notes         Z  PBG Al
#----------   ---- ---------  - --------------- -  --- --
Cogrikonur    0101 C8C5446-9    Fl Ni              120 Im";

        request.AddParameter("text/plain", body, ParameterType.RequestBody);
        RestResponse response = client.Execute(request);

        if (response.IsSuccessful) {
            result = response.RawBytes;
        }
        else {
            result = null;
        }

        return result;
    }
}

Test Application Console:

Console.WriteLine("Hello, World!");

Console.WriteLine("Testing For Success");

Connection goodConnection = new Connection(false);
byte[] image = goodConnection.CreateSectorMapTest();

if (image != null) {
    Console.WriteLine("Good Connection Worked");
}
else {
    Console.WriteLine("Good Connection Failed");
}

Console.WriteLine("Testing For Fail");
Connection badConnection = new Connection(true);
byte[] imageBad = badConnection.CreateSectorMapTest();

if (imageBad != null) {
    Console.WriteLine("Bad Connection Worked");
}
else {
    Console.WriteLine("Bad Connection Failed");
}

Console.ReadLine();

Console.WriteLine("Finished");

Output of Console App: Hello, World! Testing For Success Good Connection Worked Testing For Fail Bad Connection Fail

Expected behavior

I would expect that the two constructors should provide the same functionality.

Desktop (please complete the following information):

• OS: [e.g. Linux]
• .NET version [e.g. .NET 6]
• Version [e.g. 107.0.3.1-alpha.0.24] + in Clone of current repo