The problem, as I see it, is the difference between Hibernate passing NULL and Hibernate passing nothing. Because there is a difference. Or at least conceptually. If Hibernare passes the NULL explicitly to the database, the procedure argument default will be ignored.

Either way this works as designed and as intended. It cannot, by definition, be a bug. So I will reclassify this as an enhancement request and mention it on the hibernate-dev list for further discussion here.