Why does JsonIdentityInfo only set the id for some objects in the array?

I need to understand a problem that happens to me when I use the JsonIdentityInfo notation of the Jackson library. I noticed that for some objects in the return list the entire object does not appear but only the id. Can any of you tell me why?
Below is my code and return data.

    @Data
@Entity
@Table(name = "attivita", schema = "public")
@AllArgsConstructor @NoArgsConstructor
@JsonIgnoreProperties({"hibernateLazyInitializer", "handler"})
public class Attivita implements Serializable {
@Id 
@GeneratedValue(strategy = GenerationType.IDENTITY)
@Column(name = "id", nullable = false)
private Long id;
@JsonIdentityInfo(generator = ObjectIdGenerators.PropertyGenerator.class, property = "id")
@OneToMany(mappedBy = "attivita", cascade = CascadeType.ALL, fetch = FetchType.LAZY, orphanRemoval = true)
private List<Lavorazione> listaLavorazioni;
}
@Data
@Entity
@Table(name = "lavorazione", schema = "public")
@AllArgsConstructor @NoArgsConstructor
@JsonIgnoreProperties({"hibernateLazyInitializer", "handler"})
public class Lavorazione implements Serializable {
//ATTRIBUTI
@Id 
@GeneratedValue(strategy = GenerationType.IDENTITY)
@Column(name = "id", nullable = false)
private Long id;
@JsonIdentityInfo(generator = ObjectIdGenerators.PropertyGenerator.class, property = "id")
@ManyToOne(targetEntity = Attivita.class, fetch = FetchType.LAZY, optional = false)
@JoinColumn(name="fk_attivita", nullable = false, updatable = false)
private Attivita attivita;
}

Below is the response received, where it can be noted that for the first object (id=1) I have the "activity" object inside, while only the id=3 appears in the second.

Thank you all.

> I noticed that for some objects in the return list the entire object
> does not appear but only the id.

An explanation of this behaviour is contained in the JsonIdentityInfo documentation where it is specified that serialization is done by serializing the first instance as full object and object identity, and other references to the object as reference values, so for the first match the the object will be fully serialized while others matches will be referred as id. In your example if this behaviour has not been adopted you would have had an infinite recursion issue due to the circular reference of Attivita and Lavorazione classes.