Bi-directional relation in hibernate is failing to be cascadelly saved

Question

I am using a Spring-Boot (2.3.3) application with Hibernate (5.4.20) and JPA repository on MySql DB, and I have a Bi-directional relationship between an entity:

@Entity
@Getter
@Setter
@NoArgsConstructor
@AllArgsConstructor
@SuperBuilder
@EqualsAndHashCode(callSuper = true)
@DynamicUpdate(DBConfig.dynamicUpdate)
@Table(name = "entity")
public class Entity implements Serializable {

    @GeneratedValue(strategy = IDENTITY)
    @Column(updatable = false, columnDefinition = "BIGINT UNSIGNED NOT NULL")
    @Id
    private Long id;
    
    ...

    @ManyToOne
    @JoinColumn(name = "entity_group_id")
    private EntityGroup entityGroup;
}

which is connected "many to one" to a group entity

@Entity
@Getter
@Setter
@NoArgsConstructor
@AllArgsConstructor
@SuperBuilder
@EqualsAndHashCode(callSuper = true)
@DynamicUpdate(DBConfig.dynamicUpdate)
@Table(name = "entity_group")
public class EntityGroup implements Serializable {

    @GeneratedValue(strategy = IDENTITY)
    @Column(updatable = false, columnDefinition = "BIGINT UNSIGNED NOT NULL")
    @Id
    private Long id;
    
    ...

    @OneToMany(cascade = {CascadeType.ALL})
    @JoinColumn(name = "entity_group_id")
    private Set<Entity> entities;
}

I am trying to save them both in 1 transaction, something like:

EntityGroup group = EntityGroup.builder().entities(getEntities()).build();
jpaRepository.save(group);

For some reason, Hibernate always fails to save the entities to the DB with this root error:

Column 'entity_group_id' cannot be null

I've tried all day to tweak the annotations but nothing is working, and I also saw this: Save child objects automatically using JPA Hibernate , but couldn't make it work. Can anyone tell me what am I doing wrong here?