BUG #16613: Built in suppress_redundant_updates_trigger() trigger not supressing redundant updates

The following bug has been logged on the website:

Bug reference: 16613
Logged by: Stephane Desnault
Email address: stephane(dot)desnault(at)gmail(dot)com
PostgreSQL version: 12.4
Operating system: Windows 10
Description:

A full description was a bit too long for the title. A more exact summary
is:
The built-in suppress_redundant_updates_trigger() trigger is not supressing
redundant updates after an ALTER TABLE ... ADD COLUMN...

Here are the steps to reproduce what I saw:
I run the following script in pgAdmin, with autocommit set to true.

--------------
CREATE TABLE test (id int, val text);
INSERT INTO test VALUES (1, 'one'), (2, 'two');

CREATE OR REPLACE FUNCTION am_i_touched() RETURNS trigger LANGUAGE
'plpgsql'
AS $BODY$
BEGIN
RAISE NOTICE 'Yes, I am touched!';
RETURN NEW;
END;
$BODY$;

CREATE TRIGGER az_test_suppress_redundant_update
BEFORE UPDATE ON public.test
FOR EACH ROW EXECUTE PROCEDURE suppress_redundant_updates_trigger();

CREATE TRIGGER bz_am_I_touched
BEFORE UPDATE ON public.test
FOR EACH ROW EXECUTE PROCEDURE am_i_touched();
----------------------------

Now, if I run

UPDATE test SET id = 1 WHERE id = 1;

the update is suppressed by the first trigger since the row is left
unchanged, and bz_am_i_touched() never fires, as expected. So far so good.

But then I run:

ALTER TABLE test ADD COLUMN new_col int;

and I then run UPDATE test SET id = 1 WHERE id = 1; again.

This time, the update is NOT suppressed and bz_am_i_touched() fires! PGAdmin
(v4) reports that one record was updated, not zero like the time before.

This is a one-off occurrence. Further UPDATE test SET id = 1 WHERE id = 1
work as expected... But then I tried UPDATE test SET id = 2 WHERE id = 2...
and again I have this strange behavior - the update is not suppressed.

Also, if the second trigger doesn't let the update go through
(am_i_touched() returns NULL rather than NEW), then subsequent updates are
NEVER suppressed, it's not a "one-off" behavior anymore.