Watcher doesn't receive Terminated message

Supervisor.cs

public class Supervisor : ReceiveActor, IWithUnboundedStash
{
  private readonly ISet<IActorRef> _registeredWorkers;
  private readonly ILoggingAdapter _log;
  
  public Supervisor()
  {
    _registeredWorkers = new HashSet<IActorRef>();
    _log = Context.GetLogger();
  }
  
  protected override void PreStart()
  {
    Become(Stopped);
  }
  
  private void Stopped()
  {
    Receive<ScaleDownBy>(m =>
    {
      for (var i = 0; i < m.NumberOfWorkers; i++)
      {
          var worker = _registeredWorkers.ElementAt(i);
          worker.Tell(PoisonPill.Instance);
          _registeredWorkers.Remove(worker);
      }
      _log.Info(
          $"Scaling down workers by {m.NumberOfWorkers}. Number of workers remaining: {_registeredWorkers.Count}");
    });

      Receive<RegisterWorker>(m => _registeredWorkers.Add(m.WorkerRef));

      Receive<AvailableForWork>(m => Stash.Stash());
  }
  
  public IStash Stash { get; set; }

}

SupervisorTests.cs

[TestFixture]
public class SupervisorTests : TestKit
{
    [Test]
    public void When_supervisor_is_asked_to_scale_down_it_should_terminate_workers()
    {
        var worker1 = CreateTestProbe("worker1");

        var supervisor = new SupervisorBuilder(this).Build();

        supervisor.Tell(new RegisterWorker(worker1.Ref));

        var watcher = CreateTestProbe();
        watcher.Watch(worker1);

        supervisor.Tell(new ScaleDownBy(1));

        watcher.ExpectTerminated(worker1);
    }
}