BUG #18402: Attaching a new partition doesn't reuse the prebuilt index on said partition

The following bug has been logged on the website:

Bug reference: 18402
Logged by: Gilles PARC
Email address: gparc(at)online(dot)fr
PostgreSQL version: 16.2
Operating system: Linux
Description:

Example :

First I create a list partitioned table and fill it
CREATE TABLE master
(
id integer NOT NULL,
millesime varchar(4) NOT NULL,
CONSTRAINT master_pk PRIMARY KEY (id, millesime)
) partition by list (millesime);

CREATE TABLE master_2022 PARTITION OF master FOR VALUES IN ('2022');
CREATE TABLE master_2023 PARTITION OF master FOR VALUES IN ('2023');
CREATE TABLE master_2024 PARTITION OF master FOR VALUES IN ('2024');
CREATE TABLE master_2025 PARTITION OF master FOR VALUES IN ('2025');

DO
$do$
BEGIN
FOR i IN 2022..2025 LOOP
INSERT INTO master VALUES (generate_series(1,100000), i::varchar);
END LOOP;
END
$do$;

I now list the created indexes:
\diS+ master*
List of relations
Schema | Name | Type | Owner | Table |
Persistence | Access method | Size | Description
--------+------------------+-------------------+----------+-------------+-------------+---------------+---------+-------------
public | master_2022_pkey | index | postgres | master_2022 |
permanent | btree | 3088 kB |
public | master_2023_pkey | index | postgres | master_2023 |
permanent | btree | 3088 kB |
public | master_2024_pkey | index | postgres | master_2024 |
permanent | btree | 3088 kB |
public | master_2025_pkey | index | postgres | master_2025 |
permanent | btree | 3088 kB |
public | master_pk | partitioned index | postgres | master |
permanent | btree | 0 bytes |
(5 rows)

Now I prepare a new 2026 partition with correct constraints and indexes and
attach it :

CREATE TABLE master_2026 (LIKE master INCLUDING DEFAULTS INCLUDING
CONSTRAINTS);
ALTER TABLE master_2026 ADD CONSTRAINT master_2026_ck CHECK (millesime =
'2026');
DO
$do$
BEGIN
INSERT INTO master_2026 VALUES (generate_series(1,100000), '2026');
END
$do$;
CREATE UNIQUE INDEX CONCURRENTLY master_2026_pkey ON master_2026 (id,
millesime);
ALTER TABLE master ATTACH PARTITION master_2026 FOR VALUES IN ('2026');
ALTER TABLE master_2026 DROP CONSTRAINT master_2026_ck;

But now, when I list the indexes again, I see that my unique index above is
not considered to use
and a new one is created (master_2026_pkey1).
\diS+ master*
Schema | Name | Type | Owner | Table |
Persistence | Access method | Size | Description
--------+-------------------+-------------------+----------+-------------+-------------+---------------+---------+-------------
public | master_2022_pkey | index | postgres | master_2022 |
permanent | btree | 3088 kB |
public | master_2023_pkey | index | postgres | master_2023 |
permanent | btree | 3088 kB |
public | master_2024_pkey | index | postgres | master_2024 |
permanent | btree | 3088 kB |
public | master_2025_pkey | index | postgres | master_2025 |
permanent | btree | 3088 kB |
public | master_2026_pkey | index | postgres | master_2026 |
permanent | btree | 3104 kB |
public | master_2026_pkey1 | index | postgres | master_2026 |
permanent | btree | 3104 kB |
public | master_pk | partitioned index | postgres | master |
permanent | btree | 0 bytes |
(7 rows)

why is it so ?

Bug or Feature ?
Regards
Gilles