Eclipselink EntityManager returning different instance for same entity

Question

I'm using EclipseLink JPA in a spring mvc application. According to the docs at About Persisting Objects there should be a single instance returned:

final Account a1 = entityManager.find(Account.class, 2851L);
final Account a2 = entityManager.find(Account.class, 2851L);
assert a1 == a2;

In my app a1 and a2 are not the same instance. Why does this happen?

The configuration is this:

@Configuration
@EnableTransactionManagement
@EnableJpaRepositories(basePackageClasses = Application.class)
@EnableJpaAuditing(auditorAwareRef = "springSecurityAuditorAware")
class JpaConfig implements TransactionManagementConfigurer {
    private static final Logger LOGGER = LoggerFactory.getLogger(JpaConfig.class);

    @Autowired
    protected DataSource dataSource;

    @Bean(name="entityManagerFactory")
    @DependsOn({"flyway"})
    public LocalContainerEntityManagerFactoryBean configureEntityManagerFactory() {
        LOGGER.debug("Initialize EntityManagerFactory");
        final LocalContainerEntityManagerFactoryBean entityManagerFactoryBean = new LocalContainerEntityManagerFactoryBean();
        entityManagerFactoryBean.setDataSource(dataSource);
        entityManagerFactoryBean.setPackagesToScan("some.package");
        entityManagerFactoryBean.setJpaVendorAdapter(new EclipseLinkJpaVendorAdapter());
        entityManagerFactoryBean.setJpaProperties(getJPAProperties());
        entityManagerFactoryBean.afterPropertiesSet();
        return entityManagerFactoryBean;
    }

    protected Properties getJPAProperties() {
        final Properties jpaProperties = new Properties();
        jpaProperties.setProperty(PersistenceUnitProperties.WEAVING, "static");
        jpaProperties.setProperty(PersistenceUnitProperties.DDL_GENERATION, PersistenceUnitProperties.NONE);
        jpaProperties.setProperty(PersistenceUnitProperties.LOGGING_LOGGER, "some.package.Slf4jSessionLogger");
        jpaProperties.setProperty(PersistenceUnitProperties.LOGGING_LEVEL, "FINE");
        jpaProperties.setProperty(PersistenceUnitProperties.LOGGING_PARAMETERS, "true");
        jpaProperties.setProperty(PersistenceUnitProperties.LOGGING_TIMESTAMP, "true");
        jpaProperties.setProperty(PersistenceUnitProperties.LOGGING_SESSION, "true");
        return jpaProperties;
    }

    @Bean(name="transactionManager")
    @Override
    public PlatformTransactionManager annotationDrivenTransactionManager() {
        return new JpaTransactionManager();
    }
}

The persistence.xml is almost empty:

<?xml version="1.0" encoding="UTF-8"?>
<persistence
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns="http://java.sun.com/xml/ns/persistence"
    xsi:schemaLocation="http://java.sun.com/xml/ns/persistence http://java.sun.com/xml/ns/persistence/persistence_2_0.xsd"
version="2.0"
>
    <persistence-unit name="DefaultUnit">
    </persistence-unit>
</persistence>