subclassing JPA repository -> WrongClass exception

[beginner_]

I have abstract class AbstractCompound. Then I have classes DefaultCompound and RegistrationCompound which both extend A.

i want to perform a special type of search on all of these classes. For that I created a custom repository interface and an actual implementation for AbstractCompound:

	public class AbstractCompoundRepositoryImpl<T extends AbstractCompound>
			implements MySpecialSearchRepository<T> {

		@PersistenceContext
		private EntityManager entityManager;
		@Autowired
		private CacheManager cacheManager;    

		public AbstractCompoundRepositoryImpl() {
		}

		public AbstractCompoundRepositoryImpl(EntityManager entityManager,
				CacheManager cacheManager) {

			this.entityManager = entityManager;
			this.cacheManager = cacheManager;
		}
	   
		@Override
		public Page<T> findByMySpecialDataType(...){
			//.. complex method
		}
    }

And the actual repository:

	@Repository
	public interface AbstractCompoundRepository<T extends AbstractCompound>
			extends MySpecialSearchRepository<T>, JpaRepository<T, Long>,
			QueryDslPredicateExecutor<T> {
	}

1 service class needs this repository as it needs to query for any type of Compound. Other service classes must only query for a specific type of compound. hence I created repositories for the concrete classes:

	@Repository
	public interface TestCompoundRepository extends AbstractCompoundRepository<TestCompound> {
	}

	public class TestCompoundRepositoryImpl extends AbstractCompoundRepositoryImpl<TestCompound> {
		public TestCompoundRepositoryImpl() {
			super();
		}

		public TestCompoundRepositoryImpl(EntityManager entityManager,
				CacheManager cacheManager) {
			super(entityManager, cacheManager);
		}
	}

Now when running tests from below class:

	@RunWith(SpringJUnit4ClassRunner.class)
	@ContextConfiguration("classpath:ApplicationContext.xml")
	public class CompoundRepositoryTest {

		@Autowired
		private TestCompoundRepository testCompoundRepository;
		@Autowired
		private RegistrationCompoundRepository registrationCompoundRepository;
		
		//...
	}

I occurence extremly weird behavior:

2 tests that use registrationCompoundRepository always pass

the other 3 tests use testCompoundRepository. From these tests 1 always passes, the other 2 always fail, however which one passes seems to be somewhat random (race condition probably) The failing test throw an exception:

org.hibernate.WrongClassException: 
	Object with id: 1000 was not of the specified subclass: 
		myPackage.RegistrationCompound (loaded object was of wrong class class myPackage.TestCompound)

Note: this object is inserted into database at start of test using import.sql (anything in there is executed by hibernate on start-up).

I'm now completely lost how I can solve the issue. It seems somewhere deep within spring it uses the wrong repository (the proxy) for whatever reason. Even more obscure is that 1 test using testCompoundRepository always passes, namely the first one run.

My question is: What is the issue? How can I solve it? Should I use a completely different approach?