@PersistenceContext on two different LocalContainerEntityManagerFactoryBean throws TransactionRequiredException

I have two LocalContainerEntityManagerFactoryBeans set up, one with @Primary annotation and one without. This is because I have to communicate with two distinct databases, and if I remove the @Primary decoration I get a duplicate bean error. Both of them are constructed in the exact same way, here's one for reference:

package com.myorg.rest.config.dba;

import ...

@Configuration
@EnableJpaRepositories(
    basePackages = "com.myorg.rest.dao.dbA",
    entityManagerFactoryRef = "dbAEntityManager",
    transactionManagerRef = "dbATransactionManager"
    )
@EnableTransactionManagement
public class DbADataSourceConfig {

    @Autowired
    private EnvProperties settings;

    @Bean
    @Primary
    public DataSource prjDataSource() {
        DataSourceProperties ds = settings.getdbADatasource();
        return ds.initializeDataSourceBuilder().type(BasicDataSource.class).build();
    }

    @Bean(name = "dbAEntityManager")
    @Primary
    public LocalContainerEntityManagerFactoryBean dbAEntityManager() {
        LocalContainerEntityManagerFactoryBean em = new LocalContainerEntityManagerFactoryBean();
        em.setDataSource(prjDataSource());
        em.setPackagesToScan(new String[] { "com.myorg.model.entities.dba" });
        em.setPersistenceUnitName("dbAUnit");

        JpaVendorAdapter vendorAdapter = new HibernateJpaVendorAdapter();
        em.setJpaVendorAdapter(vendorAdapter);
        em.setJpaProperties(additionalProperties());
        // em.afterPropertiesSet();

        return em;
    }

    @Bean(name = "dbATransactionManager")
    @Primary
    public PlatformTransactionManager dbATransactionManager() {
        JpaTransactionManager transactionManager = new JpaTransactionManager();
        transactionManager.setEntityManagerFactory(dbAEntityManager().getObject());

        return transactionManager;
    }

    Properties additionalProperties() {
        Properties properties = new Properties();
        properties.setProperty("hibernate.dialect", "org.hibernate.dialect.MySQL5Dialect");
        return properties;
    }
}

Both of them are injected inside DAOs with @PersistenceContext(unitName = "dbAUnit") and @PersistenceContext(unitName = "dbBUnit"), and they are both successfully bootstrapping the corresponding EntityManager inside. I can create in both of them and then retrieve the created entity. But when I go to retrieve all entities with the following:

public List<T> findAll() {
    CriteriaBuilder criteriaBuilder = entityManager.getCriteriaBuilder();
    CriteriaQuery<T> entityQuery = criteriaBuilder.createQuery(clazz);
    entityQuery.from(clazz);
    return entityManager.createQuery(entityQuery).getResultList();
}

One of them works flawlessly and the other one doesn't (it returns 0 entities). After debugging, I've realized that one of them was issuing the insert into... sql command, while the other one wasn't. I tried to force with entityManager.flush() and got

 Caused by:
            javax.persistence.TransactionRequiredException: no transaction is in progress
                at org.hibernate.internal.AbstractSharedSessionContract.checkTransactionNeededForUpdateOperation(AbstractSharedSessionContract.java:413)
                at org.hibernate.internal.SessionImpl.checkTransactionNeededForUpdateOperation(SessionImpl.java:3398)
                at org.hibernate.internal.SessionImpl.doFlush(SessionImpl.java:1355)
                at org.hibernate.internal.SessionImpl.flush(SessionImpl.java:1350)
                at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
                at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
                at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
                at java.base/java.lang.reflect.Method.invoke(Method.java:566)
                at org.springframework.orm.jpa.ExtendedEntityManagerCreator$ExtendedEntityManagerInvocationHandler.invoke(ExtendedEntityManagerCreator.java:366)
                at com.sun.proxy.$Proxy95.flush(Unknown Source)

I then tried to decorate the DAOs with @Transactional, and further decorate each individual method with @Transactional. Also tried injecting with @PersistenceContext(unitName = "dbAUnit", type = PersistenceContextType.TRANSACTION), but all of those attempts result in the exact same error.

Why is the 'secondary' @PersistenceContext not putting the transactions in?

When using multiple transaction managers with @Transactional, you need to declare the transaction manager to use explicitly, otherwise the @Primary manager always gets selected, and your secondary EntityManager cannot possibly join its transaction.

Try using @Transactional("dbBTransactionManager") wherever the second persistence context is injected, to see if the problem goes away.