Call to detach method is not removing entity from persistence context - spring-data-jpa

Question

My environment
Java 8/Spring Data JPA 2.0.5/JPA 2.1/Eclipse Link 2.6.4/JBoss EAP 7.1
My scenario
I have three entities (#Entity): Folder, Configuration and Document.
I have three repositories (#Repository): FolderRepository, ConfigurationRepository and DocumentRepository.
I have one service (#Service): TestService.
Transactionality is only defined in TestService methods.
My Problem
If we execute 'test' method, code try to persist a 'Folder' entity in database:
If entity doesn't exist in database, then is persisted and the call to 'documentRepository.findById' works properly.
If entity already exists in database, a PersistenceException is thrown. We catch the exception and detach the entity. When we call to
'documentRepository.findById' a SQLIntegrityConstraintViolationException is thrown. Why?
I don't understand why this exception in thrown if we have detached the entity. This entity should not be in the persistence context.
My code
#Service
public class TestServiceImpl implements TestService
{
#Autowired
private FolderRepository folderRepository;
#Autowired
private ConfigurationRepository configurationRepository;
#Autowired
private DocumentRepository documentRepository;
#PersistenceContext
private EntityManager em;
#Override
#Transactional(readOnly = false)
public void test(String documentCode)
{
List<Configuration> configuration = configurationRepository.findAll();
DateFormat dateFormat = new SimpleDateFormat("yyyyMM");
Date date = new Date();
String folderName = dateFormat.format(date)
Folder folder = new Folder ();
folder.setName (folderName); //Primary key
folder.setDateCreation (date);
// We try to persist 'folder' entity using an EntityManager instance.
// We don't use 'saveAndFlush' because we don't want a merge happens.
// If constraint exception is thrown, we detach the 'folder' entity
try
{
em.persist(folder);
em.flush();
}
catch (PersistenceException e)
{
em.detach(folder);
}
// Why is a SQLIntegrityConstraintViolationException thrown here?
folder = documentRepository.findById (documentCode);
............ more code ............
}
}