What is the purpose of the @Model ...

Why is there no active transaction...

For every normal-scoped bean there is a proxy, What you see is the instantiation of two objects: one is the actual bean instance, the other one is the proxy. Both likely invoke the default constructor.

That's why it's generally considered a bad idea to do initialization in class construction code. Instead, when using managed beans (objects managed by the EE container) to perform initialisation in a @PostConstruct or @Inject annotated method.

@SessionScoped
public class SampleBean {
   private Map x;

   @PostConstruct
   protected void init() {
      x = new ConcurrentHashMap();
   }
}