Autofac Singleton Decorator constructed multiple times

i'm trying to create a simple in-memory caching mechanism for some services. I'm using the Decorator pattern, creating a cached version of the service that decorates the concrete implementation of it. By using Autofac i will register the cache decorator as SingleInstance() in order to be sure that the data stays in memory.

 builder.RegisterDecorator<IBackendOperatorServiceProxy>(
      (c, inner) =>  new CachedBackendOperatorServiceProxy(inner), 
      fromKey: "BackendOperatorService")
      .SingleInstance();

and i will register the concrete implementation as

 builder.RegisterType<BackendOperatorServiceProxy>().Named<IBackendOperatorServiceProxy>("BackendOperatorService");

The problem that i'm having is that when resolved in the webApi controller, this singleton decorator is always recreated (once per request).

I have (hopefully) correctly set-up the owin pipeline for autofac (in fact the service gets injected in the BL class called by the controller).

I really can't understand a possible reason for an issue like this. I've used SIngleInstance() different times with different projects and it always worked without issues. To further "investigate" this i've also created a fake class registered as singleton and injected it in the same controller as the decorator. It works as expected, creating just a single instance of the fake class. For completeness, i've registered this fake one as:

builder.Register<Foo>(c => new Foo(c.ResolveNamed<IBackendOperatorServiceProxy>("BackendOperatorService"))).AsImplementedInterfaces().SingleInstance();

And this is the constructor of the Cached (Decorator) service:

public CachedBackendOperatorServiceProxy(IBackendOperatorServiceProxy decoratedServiceImplementation)
{
  _decoratedServiceImplementation = decoratedServiceImplementation ?? throw new ArgumentNullException(nameof(decoratedServiceImplementation));
  _cachedElements = new Dictionary<string, CachedOperatorDto>();
}

Is there any problem or something that i'm missing with decorators and SIngleInstances?