Keeping flash via #{flash.keep.foo} does not to work, instead #{flash.foo} works

Question

I have an app running with Primefaces, JSF 2.2, Spring and Hibernate. I use JSF's Flash Scope to send a parameter with a redirection:

FacesContext.getCurrentInstance().getExternalContext().getFlash().put("loginCliente", cliente.getLogin());
return "registroCasa?faces-redirect=true";

If I write #{flash.keep.loginCliente} in registroCasa.xhtml and I reload the page the value becomes null, but if I write #{flash.loginClient} the value keeps alive. Why?

I'm using Mojarra 2.2.12.