BUG #13269: "alter constraint child_parent deferrable initially deferred" sometimes does not make FK deferred

The following bug has been logged on the website:

Bug reference: 13269
Logged by: Piotr Findeisen
Email address: piotr(dot)findeisen(at)gmail(dot)com
PostgreSQL version: 9.4.1
Operating system: Ubuntu 14.04.1
Description:

I experienced the problem in different configurations, originally using
JDBC. I managed to repro using just psql, but it may be a bit different
problem that originally experienced in my Java program.

Below follows SQL that in my opinion should fail at 'commit', i.e. after 6 s
sleep. However, when run with psql, it fails immediately, as if FK was not
made deferred.

----------------------------
create table parent (parent_id bigint primary key);

create table child (
child_id bigint primary key,
parent_id bigint constraint child_parent references parent (parent_id)
--deferrable initially deferred
);

insert into parent (parent_id) values (1);
insert into child (child_id, parent_id) values (2, 1);

alter table child alter constraint child_parent deferrable initially
deferred;

-- commit; -- helps in pgadmin

begin transaction;

delete from parent;

select pg_sleep(6);

commit;

----------------------------

Additional note: when I run the script with pgadmin, if tails immediately
too.
However, if I ad 'commit' after 'alter table', it fails after 6 s (as
expected). Strange, as "there is no transaction in progress" warning is
issued suggesting that commit did not change anything.