BUG #17240: at time zone ... ; wrong result

The following bug has been logged on the website:

Bug reference:      17240
Logged by:          Marek Läll
Email address:      marek.lall@eesti.ee
PostgreSQL version: 13.4
Operating system:   Debian
Description:

Hello!

# SELECT version();
                                                        version
                                           

------------------------------------------------------------------------------------------------------------------------
 PostgreSQL 13.4 (Debian 13.4-1.pgdg100+1) on aarch64-unknown-linux-gnu,
compiled by gcc (Debian 8.3.0-6) 8.3.0, 64-bit
(1 row)


# SET TimeZone='UTC';

# select * from pg_timezone_names where name like 'America/Los_Angeles';
        name         | abbrev | utc_offset | is_dst 
---------------------+--------+------------+--------
 America/Los_Angeles | PDT    | -07:00:00  | t
(1 row)

# select now()
 , now() at time zone 'America/Los_Angeles' as correct
 , now() at time zone '-07:00:00' as wrong;

              now              |          correct           |
wrong            
-------------------------------+----------------------------+----------------------------
 2021-10-21 09:29:07.591962+00 | 2021-10-21 02:29:07.591962 | 2021-10-21
16:29:07.591962
(1 row)

--> COLUMN "WRONG" IS EXPECTED TO HAVE THE SAME VALUE AS COLUMN "CORRECT"

# select timestamptz'2021-10-01 00:00:00 UTC' at time zone
'America/Los_Angeles' as correct_1
 , timestamptz'2021-10-01 00:00:00 UTC' at time zone '-07:00' as wrong_1
 , timestamptz'2021-10-01 00:00:00 America/Los_Angeles' at time zone
'America/Los_Angeles' as correct_2
 , timestamptz'2021-10-01 00:00:00 -07:00:00' at time zone '-07:00' as
wrong_2
 , timestamptz'2021-10-01 00:00:00 -07:00:00' at time zone
'America/Los_Angeles' as correct_3
 , timestamptz'2021-10-01 00:00:00 America/Los_Angeles' at time zone
'-07:00' as wrong_3;
 
      correct_1      |       wrong_1       |      correct_2      |
wrong_2       |      correct_3      |       wrong_3       

---------------------+---------------------+---------------------+---------------------+---------------------+---------------------
 2021-09-30 17:00:00 | 2021-10-01 07:00:00 | 2021-10-01 00:00:00 |
2021-10-01 14:00:00 | 2021-10-01 00:00:00 | 2021-10-01 14:00:00
(1 row)

--> COLUMNS "WRONG_<N>" ARE EXPECTED TO HAVE THE SAME VALUE AS COLUMN
"CORRECT_<N>"