The Id generator scope has changed from local scoped (or qualified by entity name, I can't remember) to global scope in strict adherence to the JPA (new TCK test).
http://in.relation.to/2018/02/07/hibernate-orm-5213-final-release/

This is creating migration trouble on large applications. https://twitter.com/aczar0/status/981084503702736896

Consider adding a legacy flag that can be easily set by a user so he can migrate at his/her own pace.