@Chris Cranford you're wellcome. In response to your comments:

1. B relation is null initialized in class A definition as best practice and the DDL is auto generated by Hibernate
2. As you can see in the debug log Envers populates first A_AUD and then A_B_AUD. The problem is that a.getB() may be null and A_B_AUD doesn't allow null relation by autogenerated constraint in field B

I hope this helps