detached entity passed to persist - on findByMethod

I understand the error message and i know how to solve it, but i want to know why it occurs in this specific place especially on a find method. I created a mini example for this.

I have three entitys:

@Entity
data class Animal(
    var name: String,
    @ManyToOne(cascade = [CascadeType.ALL]) val zoo: Zoo) {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    var id: Int = -1
}

@Entity
data class Zoo(
    var city: String,
    @OneToMany(cascade = [CascadeType.ALL]) val employee: MutableList<Person>) {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    var id: Int = -1
}

@Entity
data class Person(var age: Int) {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    var id: Int = -1
}

Service:

   @Transactional
    fun save(name:String, city: String){
        repo.findByZooCity(city).ifPresent {
            it.zoo.employee.add(Person(22))
        }
        repo.findByZooCity("OTHER STRING!!!!").ifPresent { println("FOUND!") }
        repo.save(Animal(name, Zoo(city, mutableListOf(Person(33)))))
    }

Repo:

interface AnimalRep: JpaRepository<Animal, Int>{

    fun findByZooCity(name: String): Optional<Animal>
}

Call:

  animalService.save("Zoo1", "Animal1")
  animalService.save("Zoo1", "Animal1")

Exception: On the second call i get a "detached entity passed to persist: com.example.Person" on repo.findByZooCity("OTHER STRING!!!!"). I know this happens because i add a "detached" person before. But WHY it occurs on a findBy? (Even its not in results?)

Is there some dirty check?

Thank you for your time and help.