Given the following entities:

@Entity
public class Bar {
    @Id @OneToOne
    private Foo foo;
    ...
}

@Entity
public class Foo implements Serializable {
	@Id
	private Long id;
	@OneToOne(mappedBy = "foo")
	private Bar bar;
    ...
}

When the following query is executed:
"SELECT f FROM Foo f"

Bar objects that get returned have a null ID (i.e., bar#foo == null)

This happens because the fix for HHH-6813 Closed sets ToOne#referenceToPrimaryKey for Bar#foo to false [1], which ultimately leads to the ID not being resolved. [2]

[1] https://github.com/hibernate/hibernate-orm/blob/master/hibernate-core/src/main/java/org/hibernate/cfg/OneToOneSecondPass.java#L227
[2] https://github.com/hibernate/hibernate-orm/blob/master/hibernate-core/src/main/java/org/hibernate/type/EntityType.java#L428-L439