I guess ther eis a regression in Hibernate 5.3.x concerning inheritence and composite primary keys.
Problem is that a NullPointerException occurs when a method is called from the constructor of a (Byte Buddy) proxied abstract entity class.
I will reference a GitHub fork, but want to create the issue first in order to get an issue number.
In this fork you can find the JPAUnitTestCase. You should reproduce the bug with this tets case.