I don’t know how this could have worked in Hibernate 5, you’d have to show me a reproducer for that, but since Kotlin creates a new field for every overridden property, you have to use property access with Hibernate. For Kotlin, this usually means that you have to prefix your annotations with @get: i.e. @get:Id etc. so that Hibernate uses Getter/Setter for interacting with objects instead of fields.