when `.copy()`ing a data class that inherits from an open class, the properties/constructor values from the open class are not copied. I understand why, but what is the proper way to do this then?

Make the base class an interface

Seems I cannot use (JPA) annotations on fields in an interface, though

Too bad

data classes for JPA are usually a bad idea anyways, see e.g. https://vladmihalcea.com/how-to-implement-equals-and-hashcode-using-the-jpa-entity-identifier/