unity.cs

using Unity;
using Unity.Lifetime;

namespace UnityFactory
{
    class Program
    {
        static void Main(string[] args)
        {
            var container = new UnityContainer();

            container.RegisterFactory<IFoo>(x =>
            {
                // This fails because x is not the container/child that is trying to resolve of IFoo, but rather the parent container that did the RegisterFactory
                return new Foo(x.Resolve<IBar>());
            }, new ContainerControlledLifetimeManager());

            var child = (container as IUnityContainer).CreateChildContainer();
            child.RegisterType<IBar, Bar>(new ContainerControlledLifetimeManager());
            
            // This fails
            var bar = child.Resolve<IFoo>();
        }
    }

    public interface IFoo { }

    public class Foo : IFoo
    {
        public Foo(IBar bar)
        {

        }
    }

    public interface IBar {}
    public class Bar : IBar {}
}

Search issues in DI repo, you'll find reason for that