When using an interface field in the mapped element as mapped-by field, the Set of the element-holder (i.e. the object having the Set field) behaves incorrectly:
* Set.isEmpty() returns true (even though there is an element).
* Set.size() returns 0 (even though there is an element).
* After calling Set.iterator(), isEmpty() returns false (correct), but the Set contains more elements than there should be (Set.size() returned 2 in my test even though there is only 1 element).