When it's first call, I think it is not a bug. Could you call the event twice? The Document describes this resason. I will try looking for it.
This is intentional (although probably not very clear in the javadoc).
You can register a listener instead, it will be called on creation as well as on modification: http://docs.jboss.org/infinispan/5.1/apidocs/org/infinispan/notifications/cachelistener/event/CacheEntryModifiedEvent.html
It is a little hard to see if the entry was created or modified though, you need to check if the value is null when isPre()==true, but you can only access the actual value when isPre()==false.