Deleting entities is not happening in the right order.
According to documentation: all entity deletions shall happen in the same order the corresponding objects were deleted using Session.delete()

When property,
hibernate.jdbc.batch_size = 20,
hibernate is using BatchingBatch, which at line 86, decides whether the statements shall be executed or not:
_if ( batchPosition == batchSize )

_

Given
FOO and BAR entities
both with InheritanceType.JOINED and
FOO having a ManyToOne relation to BAR

When
The following two lines are executed:
entityManager.remove(foo);
entityManager.remove(bar);

Then
Hibernate doesn't execute the statement to remove FOO, but later
It does execute the statement to remove BAR
And as a result:
delete from BAR where id=2 throws
Referential integrity constraint violation: "FKJFU6P0IRDHEYXGG5UDT0EAJAH: PUBLIC.FOO FOREIGN KEY(BAR_ID) REFERENCES PUBLIC.BAR(ID) (2)";

Please check the attached test application.
Just run mvn install.