This looks like a bug to me. The code should also be checking the interfaces of all the super classes of the AnnotationBasedGenerator. That code should probably use GenericsHelper.typeArguments(AnnotationBasedGenerator.class, annotationBased.getClass()) instead to determine the type variable assignment.
A pull request with an accompanying test is very much appreciated. For details about how you can make Jira work for you, take a look into these discussion mentioned here: Request for Jira issue creation permission (Hibernate ORM)