The spec for EntityManager.setProperty(String propertyName, Object value) says: "Set an entity manager property or hint. If a vendor-specific property or hint is not recognized, it is silently ignored."

This means that vendor-specific (i.e. custom) properties will be silently ignored, but Hibernate Session implementation seems to interpret ignored as discarded (see SessionImpl.java#L3571) for no reason.

A developer should have means to set arbitrary objects as properties of a session, to set them at some point and retrieve their values later during the same session. A simple use case:

@Stateless
public class DAO {
    @Inject LocaleResolver localeResolver;
    @PersistenceContext entityManager;

    // ... DAO methods

    EntityManager getEntityManager() {
        this.entityManager.setProperty("current-locale", this.localeResolver.resolve());
        return this.entityManager;
    }
}

and then retrieve "current-locale" value in an event listener (e.g. PreInsert and PostLoad).