How to recognize an update to OneToMany-relation done on the many side?

Question

I've got a person-taxcase one to many relation:

in Person-class:

@OneToMany(mappedBy = "person")
private List<TaxCaseEntity> taxCases = new ArrayList<>();

TaxCase-class:

@ManyToOne(targetEntity = PersonEntity.class)
private PersonEntity person;

. Person can have multiple tax cases. When I create a new tax case like this:

 @Test
 @Transactional
public void test() {
    UUID personId = UUID.randomUUID();

    PersonEntity personEntity = createPersonEntity(mandantId);

    personRepository.save(personEntity);

    TaxCaseEntity taxCase = createTaxCase();
    taxCase.setPerson(personEntity);

    taxCaseRepository.save(taxCase);

    PersonEntity foundEntity = personRepository.findById(personId).get();
    assertThat(foundEntity.getId()).isEqualTo(personId);
}

Found entity has zero tax cases. However, when I create it like this:

  PersonEntity personEntity = createPersonEntity(UUID.fromString("05e710ea-f8a4-40ba-b67c-8a685f35fc84"));
    EntityManager em = entityManagerFactory.createEntityManager();

    em.getTransaction().begin();
    em.persist(personEntity);
    em.getTransaction().commit();

    TaxCaseEntity taxCaseEntity = createTaxEntity()
                    .setPerson(personEntity);

    em.getTransaction().begin();
    em.persist(taxCaseEntity);
    em.getTransaction().commit();
    em.close();

The subsequent call to personRepository.findById(..) yields a person entity instance with one tax case in the tax case list.

The question is - how can I make sure that the find-Method yields a person entity instance with the taxcase (the first snippet?) Do I have to use explicit commits or some force reloading?