Why interface type members are interpreted as the enclosing interface while creating an XmlSerializer in dotnet?

Question

The title says it all, consider this program:

using System;
using System.Xml.Serialization;

public interface ITest
{
    public sealed class Test //: ITest
    {
        public string TestString { get; set; }
    }
    
    string TestString { get; }
}

public sealed class Test : ITest
{
    public string TestString { get; set; }
}

public class Program
{
    public static void Main()
    {
        var serializer1 = new XmlSerializer(typeof(Test));
        Console.WriteLine($"{nameof(serializer1)} created.");
        var serializer2 = new XmlSerializer(typeof(ITest.Test));
        Console.WriteLine($"{nameof(serializer2)} created.");
    }
}

The output is:

serializer1 created.
Unhandled exception. System.NotSupportedException: Cannot serialize interface ITest.
   at System.Xml.Serialization.TypeDesc.CheckSupported()
   at System.Xml.Serialization.TypeScope.GetTypeDesc(Type type, MemberInfo source, Boolean directReference, Boolean throwOnError)
   at System.Xml.Serialization.TypeScope.ImportTypeDesc(Type type, MemberInfo memberInfo, Boolean directReference)
   at System.Xml.Serialization.TypeScope.GetTypeDesc(Type type, MemberInfo source, Boolean directReference, Boolean throwOnError)
   at System.Xml.Serialization.ModelScope.GetTypeModel(Type type, Boolean directReference)
   at System.Xml.Serialization.XmlReflectionImporter.ImportTypeMapping(Type type, XmlRootAttribute root, String defaultNamespace)
   at System.Xml.Serialization.XmlSerializer..ctor(Type type, String defaultNamespace)
   at System.Xml.Serialization.XmlSerializer..ctor(Type type)

But ITest.Test is not an interface, it is a (concrete) sealed class!!

What's going on? is it a bug maybe?