After some further digging, I found out that the error will only occur if I use the EntityManager#merge over EntityManager#persist, so I added another test case which succeeds in this special scenario.

This is also the reason why the org.hibernate.jpa.test.inheritance.InheritanceTest does not fail: It uses EntityManager#persist.

Due to the fact that my test fails while merging the first child in the inheritance hierarchy, I think this does not only apply to multiple inheritance but to simple inheritance, too.

This clearly seems to be a Hibernate EntityManager bug. Can you confirm this?