BUG #17689: Two UPDATE operators in common table expressions (CTE) perform not as expected

The following bug has been logged on the website:

Bug reference:      17689
Logged by:          Eugene Pliskin
Email address:      eugene.pliskin@gmail.com
PostgreSQL version: 14.5
Operating system:   Windows 10
Description:

The sample code below contains two UPDATE steps in a chain of common table
expressions (CTE).
First UPDATE step "t2b" modifies a row in a sample table.
Later down an unconditional UPDATE step "t5" fails to affect that row.
If step "t2a" is uncommented instead of "t2b" then step "t5" performs as
expected .

create table table1(id int, value1 int);
insert into table1 values
    (1, 10),
    (2, 20)
;
select * from table1 order by 1;
-- id  value1
-- 1    10       
-- 2    20       

do $$declare -- anonymous plpgsql block
  n2 int; n5 int;
begin
    with t1 as ( -- t1 has one row with (id = 1)
        select * from table1 where id = 1
    ), t2 as ( 
-- Experiment (t2a) does nothing:
--         select * from t1
-- Experiment (t2b) updates the first row of table1 with (value1 = 11):
        update table1 set value1 = 11 
        from t1
        where table1.id = t1.id
        returning table1.id
    ), t3 as (
        select count(*) as n2 from t2  -- n2 = 1
    ), t4 as (  -- t4 = table1, two rows
        select * from table1
    ), t5 as (   -- expect to update both rows of table1 with (value1 = 22)
and so it does after t2a
                 -- but only one row gets updated after t2b
        update table1 set value1 = 22 
        from t4
        where table1.id = t4.id
        returning table1.id
    ), t6 as (
        select count(*) as n5 from t5  -- expect n5 = 2
    ) select t3.n2, t6.n5
      from t3, t6
      into n2, n5;
  raise notice 'n2: %, n5: %', n2, n5; -- expected (n2: 1, n5: 2) but have
(n2: 1, n5: 1) in t2b
end$$ ;
-- t2a: (n5 = 2) as expected
-- t2b: unexpected (n5 = 1)

select * from table1 order by 1;
-- t2a: expected result, both rows have (value1 = 22)
-- id  value1
-- 1    22       
-- 2    22       
--
-- t2b: unexpected (value1 = 11) in the first row
-- id  value1
-- 1    11       
-- 2    22       
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