IdentifierGenerationException: attempted to assign id from null one-to-one property + #MapsId + OneToOne bidirectional - jpa-2.0

Question

I'm getting error
org.hibernate.id.IdentifierGenerationException: attempted to assign id from null one-to-one property.
The code snippets:
User.java
#Entity
#Table(schema = "public", name = "user_01")
public class User {
#Id
#GeneratedValue(generator = "UUID")
#GenericGenerator(name = "UUID", strategy = "org.hibernate.id.UUIDGenerator")
#Column(name = "id", updatable = false, nullable = false)
private UUID id;
#OneToOne(cascade = CascadeType.ALL)
#PrimaryKeyJoinColumn
#JoinColumn(name = "user")
private UserInformation userInformation;
UserInformation .java
#Entity
#Table(schema = "public", name = "user_information_06")
public class UserInformation {
#Id
#Column(name = "id")
private UUID id;
#MapsId
#OneToOne(mappedBy = "userInformation")
private User user;
UserInformationService.java
// CREATE
public UserInformationBean createUserInformation(UserInformationBean userInformationBean) {
if(userInformationBean == null || userInformationBean.getUserId() == null)
return null;
User user = userRepository.findById(userInformationBean.getUserId()).orElse(null);
if(user == null)
return null;
UserInformation userInformation = user.getUserInformation();
if(userInformation != null) {
System.err.println("User Information exists !! " + userInformation.getId());
return null;
}
userInformation = new UserInformation();
userInformation.setFirstname(userInformationBean.getFirstname());
userInformation.setLastname(userInformationBean.getLastname());
userInformation.setGender(userInformationBean.getGender());
userInformation.setDateOfBirth(userInformationBean.getDateOfBirth());
userInformation.setProfession(userInformationBean.getProfession());
userInformation.setUpdatedOn(Timestamp.valueOf(LocalDateTime.now()));
user.setUserInformation(userInformation);
userInformation.setUser(user);
user = userRepository.save(user);
return userInformation.toBean();
}

Answer 1

After changing the below:
user = userRepository.save(user);
To:
userInformation = userInformationRepository.save(userInformation);
It works fine! Still want to know the reason and why it can't be persisted via User entity.