If an Entity Bean has a @PreUpdate listener, the listener is not called in the context of the caller (session bean) in case of Container Managed Transactions.
As a result, an jndi lookup in the callback for 'java:comp/EJBContext' returns always null. The @PreUpdate callback should run in the context of the caller.

@Entity
@EntityListeners({ MyListener.class })
public class MyEntity {
... 
}

public class MyListener {
 
    @PreUpdate
    @PrePersist
    public void onEntityCallback(Object entity) {
          EJBContext ctx;
 
          ctx = createInitialContext().lookup("java:comp/EJBContext");
          System.out.println(ctx.getCallerPrincipal().getName());
    }
}

@Stateless
public class MyServiceBean implements MyServiceRemote {

    @Override
    public void updateEntity(MyEntity entity) {
        entityManager.merge(entity);
    }
}

This listener works fine on @PrePersist (Container- and Bean Managed Transactions) but fails on @PreUpdate with a NullPointerException if container managed transactions are used.

Workaround: use User Transactions instead of Container Managed Transactions