OptimisticLockException not thrown when version has changed - websphere

Question

I've created a simple EJB application that uses JPA for persistence and have a problem whereby optimistic locking is not functioning as I would have expected.
The application contains a class named Site which defines the model for a table named SITE in the database. The SITE table contains a column named ROW_VERSION which is referenced in the Site class using the #version annotation.
Whenever the record is updated, the ROW_VERSION is incremented by 1. So far, so good.
The problem arises when the row has changed in the time between the application reading the row using the EntityManager find method and the row being updated by the EntityManager merge method. As the ROW_VERSION for the row has been incremented by 1 and therefore is not the same as when the EntityManager find method was called, I would expect an OptimisticLockException to be thrown, but instead the changes are written to the table and in turn overwriting the changes made by the other process.
The application is running on WebSphere 8.5 and is using OpenJPA provided by the container.
Have I mis-understood how optimistic locking is supposed to work or is there something else that I need to do to make the OptimisticLockException occur?
The Site class is as follows:
#Entity
#Table(name="SITE")
public class Site {
#Id
#Column(name="SITE_ID")
private int id;
#Column(name="SITE_NAME")
private String siteName;
#Column(name="SITE_ADDRESS")
private String address;
#Column(name="ROW_VERSION")
#Version
private long rowVersion;
//getters and setters
}
The application makes uses of the Generic DAO wrapper class to invoke the EntityManager methods. The contents of the class are as follows:
public abstract class GenericDAO<T> {
private final static String UNIT_NAME = "Test4EJB";
#PersistenceContext(unitName = UNIT_NAME)
private EntityManager em;
private Class<T> entityClass;
public GenericDAO(Class<T> entityClass) {
this.entityClass = entityClass;
}
public T update(T entity) {
return em.merge(entity);
}
public T find(int entityID) {
return em.find(entityClass, entityID);
}
//other methods
}
Update - I've done some more investigation and have found this http://pic.dhe.ibm.com/infocenter/wasinfo/v8r0/index.jsp?topic=%2Fcom.ibm.websphere.nd.multiplatform.doc%2Finfo%2Fae%2Fae%2Fcejb_genversionID.html but even when I've added the #VersionColumn and #VersionStrategy annotations I still cannot get the OptimisticLockException to be thrown.