I'm seeing something similar in Hibernate 5.1.4.

My schema looks like this:

public abstract class Super {

    private Thing thing;

    // "thing" is optional
    @ManyToOne
    public Thing getThing() {
        return this.thing;
   }
    public void setThing(Thing x) {
        this.thing = x;
   }
}

public class SubA extends Super {
    // "thing" is optional
}

public class SubB extends Super {

    // "thing" is mandatory
    @Override
    @NotNull
    public Thing getThing() {
        return super.getThing();
    }
}

So the thing property is optional for SubA and mandatory for SubB.

Here's the problem. This query always returns null even if the object exists:

this.entityManager.find(SubA.class, id);

The reason is that Hibernate generates an INNER JOIN to the Thing table, even though that reference is optional for SubA instances.

It's as if the @NotNull constraint is going "up" the class hierarchy from SubB, to Super, and back down (?) affecting sibling class SubA.

It seems like a simple rule has been broken: JSR 303 validation constraints should apply downward in the class hierarchy, but never upward!