• Entity Master has a simple @Id and @OneToMany relationship with Detail
• Entity Detail has a cascaded key using @IdClass and a @ManyToOne relationship with Master

Master m = new Master('masterkey');
Detail d = new Detail(m, 0);
em.createQuery("select count(d) from Detail d where d=:d")
  .setParameter("d", d)
  .getSingleResult()

leads to em.contains(m) == true which results in insert into table at flush/commit time
See attached test case