OrphanRemoval with cascade of PERSIST and MERGE

Question

I have two objects with @OneToOne relation:

@Entity
public class User implements Serializable {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "id_user")
    private Long id;

    @OneToOne(cascade = CascadeType.MERGE, orphanRemoval=true)
    @JoinColumn(name = "id_details")
    private UserDetails details;
//...username, password, email
}

@Entity
public class UserDetails implements Serializable {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "id_user_details")
    private Long id;
//...firstname, lastname, address
}

First of all, I save the user with UserDetails:

public class Main {
    public static void main(String[] args) {
        EntityManagerFactory emf = Persistence.createEntityManagerFactory("pu");
        EntityManager em = emf.createEntityManager();

        em.getTransaction().begin();
        UserDetails details = new UserDetails("John", "Doe", "NYC 11");
        User user = new User("uname", "pass", "mail", details);
        em.merge(user);
        em.getTransaction().commit();

        em.close();
        emf.close();
    }
}

Everything works fine, because there is a cascading MERGE, so both objects are saved. When I run another class:

public class MainUpdate {
    public static void main(String[] args) {
        EntityManagerFactory emf = Persistence.createEntityManagerFactory("pu");
        EntityManager em = emf.createEntityManager();

        User user = em.find(User.class, 1L);
        user.setDetails(null);

        em.getTransaction().begin();
        em.merge(user);
        em.getTransaction().commit();

        em.close();
        emf.close();
    }
}

I expected the user id_details to be set to null in the database (and it will), but since there is orphanRemoval = true, I also expected the orphan row to be removed from the userdetails table, but that is not the case.

Interestingly, when I install CascadeType PERSIST instead of MERGE, the orphan is removed. Can someone explain why PERSIST works with orphanRemoval, but MERGE does not?

GitHub repo: https://github.com/slwch/onetoone-orphan

Tested with Hibernate 5.0.6 and 4.1.8