As i said, the JPA spec defines JPQL and it does NOT say that this query should work, nor does it use the term 'multi morph' anywhere.

DataNucleus allows JPQL to be used with JDO. But that query (candidate class using "subclass-table") is not supported like that, whether using JDOQL or JPQL. As also said, if you want that supported you need to contribute it