BUG #17823: Generated columns not always updated correctly

The following bug has been logged on the website:

Bug reference: 17823
Logged by: Hisahiro Kauchi
Email address: hisahiro(at)freemind(dot)co(dot)jp
PostgreSQL version: 15.2
Operating system: CentOS 7
Description:

I found that the generated columns are sometimes not updated.

1. Create a table with a generated column, and insert a row:

test=# create table test(id serial primary key, a int, b int generated
always as (a + 1) stored);
CREATE TABLE
test=# insert into test(a) values (1);
INSERT 0 1
test=# select * from test;
id | a | b
----+---+---
1 | 1 | 2
(1 row)

2. Start Transaction A and update the row:

==== Transaction A ====

test=# begin;
BEGIN
test=*# update test set a=2 where id=1;
UPDATE 1
test=*# select * from test;
id | a | b
----+---+---
1 | 2 | 3
(1 row)

3. Before committing transaction A, start Transaction B to update the same
row:

==== Transaction B ====

test=# begin;
BEGIN
test=*# update test set a=3 where id=1;

(Waiting for Transaction A to commit)

4. Commit Transaction A:

==== Transaction A ====

test=*# end;
COMMIT

5. The UPDATE of Transaction B is executed:

==== Transaction B ====

UPDATE 1
test=*# end;
COMMIT

6. Check the result:

test=*# select * from test;
id | a | b
----+---+---
1 | 3 | 3
(1 row)

The generated column "b" should be updated correctly to reflect the new
value of "a" (i.e., b=4).