Observe ReactiveObject.Changed of inherited class may or may not work

I try to create a base class, which inherits from ReactiveObject. This class needs to check if any property, including properties of further inheritations, has changed.

My main problem is, that I can't get any reliable testresults! The provided test MAY be green. Running it with VS Live Unit Testing, brings it in red. Moving the classes out of the testproject into a library may or may not get this test green.

So far, the only way to keep it green, is to uncomment this line in the inherited class.

//  Changed.Subscribe(Console.WriteLine);

   [TestClass]
   public class StateBaseTests
   {
      #region Tests

      [TestMethod]
      public void State_ChangesToChanged_PropertyChanged()
      {
         var _item = new MyTest();

         _item.Surname = "Testname";

         Assert.AreEqual(Statetype.Changed, _item.State);
      }

      #endregion

      #region SubClasses

      public enum Statetype
      {
         Added,
         Changed,
         Deleted,
         Detached,
         Unchanged
      }

      public abstract class StateBase : ReactiveObject
      {
         protected StateBase()
         {
            State = Statetype.Unchanged;

            Changed.Select(prop => prop.PropertyName)
                   .Subscribe(UpdateZustand);
         }

         protected void UpdateZustand(string propertyName)
         {
            if(State == Statetype.Unchanged)
               State = Statetype.Changed;
         }

         [Reactive]
         public Statetype State { get; set; }
      }

      private class MyTest : StateBase
      {
         public MyTest()
         {
            //  Changed.Subscribe(Console.WriteLine);
         }

         [Reactive]
         public string Name { get; set; }

         [Reactive]
         public string Surname { get; set; }
      }
   }