We have following structure in our project:
abstact class X

abstract class Y

class Z

X, Y, Z are Entities. X Entity has Inheritance.JOINED and @DicriminatorColumn("INTH_TYPE);
On persistence record is populated correct with generated discrimiantor column in table defined in X Entity.
Exception appears on querying records from Db using Y Entity as EntityClass from EntityManager. Here is a test on Maven.