Minimal test-case can be found here : https://github.com/hibernate/hibernate-test-case-templates/commit/d73b8d26c4a1557042976c534a8dd297966048d9

cd orm/hibernate-orm-5
mvn clean verify
# OK
cd ../../orm/hibernate-orm-6
mvn clean verify
# failure
Tests in error:
  hhh16641Test(org.hibernate.bugs.JPAUnitTestCase): Cannot invoke "org.hibernate.annotations.common.reflection.XClass.getName()" because the return value of "org.hibernate.annotations.common.reflection.XProperty.getMapKey()" is null
  hhh16641Test(org.hibernate.bugs.JPAUnitTestCase): Cannot invoke "jakarta.persistence.EntityManagerFactory.close()" because "this.entityManagerFactory" is null

For both orm/hibernate-orm-5 and orm/hibernate-orm-6 :

• WithoutGenericEntity → TargetEntityWithoutGeneric : works OK with Hibernate 5 / 6
• WithGenericConcrete → TargetEntityWithGeneric : works OK with Hibernate 5 ; fails with Hibernate 6

Unit test only tries to startup EntityManagerFactory ; no other operation.

If WithGenericConcrete / TargetEntityWithGeneric is removed from entities : OK

If @OrderColumn is removed on generic collection : OK