Why does the persisting order in jpa matter?

Question

I'm using Spring boot 2.0.1, I have the following entities :

    @Entity
    public class Session {
        @Id
        @GeneratedValue(strategy=GenerationType.AUTO)
        private int id;
        @ManyToOne
        private Groupe groupe;
        ...

and

@Entity
public class Groupe {
    @Id
    @GeneratedValue(strategy=GenerationType.AUTO)
    private Integer id;
    @OneToMany(mappedBy="groupe")
    private List<Session> sessions;
    ...

I'm wondering why if I persist Group before Session it works, but not if I do the opposite, here is an example : Working case :

Groupe groupe1= new Groupe("groupe1",professor);
Session session1 = new Session(new Date(),false,groupe1);
Session session2 = new Session(new Date(),true,groupe1);
groupeRepository.save(groupe1); // <-----
sessionRepository.save(session1);
sessionRepository.save(session2);

Not working case :

Groupe groupe1= new Groupe("groupe1",professor);
Session session1 = new Session(new Date(),false,groupe1);
Session session2 = new Session(new Date(),true,groupe1);
sessionRepository.save(session1);
sessionRepository.save(session2);
groupeRepository.save(groupe1); // <-----

It gives me the following exception :

org.hibernate.TransientPropertyValueException: object references an unsaved transient instance - save the transient instance before flushing : gestionAbs.bean.Session.groupe -> gestionAbs.bean.Groupe 

I was expecting the opposite, since the relation is mapped by Session, Session is "the owner" of the relationship, persisting session should persist groupe, but it seems that I'm wrong, anyone can explain to me why please?

Thank you