BUG #17361: Unique index constraint inconsistence

The following bug has been logged on the website:

Bug reference:      17361
Logged by:          Роман Григорович
Email address:      atzedus@gmail.com
PostgreSQL version: 12.7
Operating system:   Ubuntu 12.7-0ubuntu0.20.04.1
Description:

I has a table in database with this structure, and this table have a unique
complex index named "parts_stock_stat_lower_stock_type_id_idx":

db=# \d parts_stock_stat
                          Table "public.parts_stock_stat"
 Column     |  Type   | Collation | Nullable |             Default
---------------+---------+-----------+----------+----------------------------------
 id               | integer |           | not null | generated by default as
identity
 name        | text       |           | not null |
 qty            | integer  |           | not null |
 stock_type_id | integer |           | not null |
Indexes:
    "parts_stock_stat_pkey" PRIMARY KEY, btree (id)
    "parts_stock_stat_lower_stock_type_id_idx" UNIQUE, btree (lower(name),
stock_type_id)

Following reindex command failed:
db=# reindex table parts_stock_stat;
ERROR:  could not create unique index
"parts_stock_stat_lower_stock_type_id_idx"
DETAIL:  Key (lower(name), stock_type_id)=(ha_hr/50, 4) is duplicated.

I was try to find problem rows with following:
db=# SELECT name::bytea, id, name, stock_type_id FROM parts_stock_stat WHERE
lower(name) = 'ha_hr/50' AND stock_type_id = 4;
        name        |  id   |   name   | stock_type_id
--------------------+-------+----------+---------------
 \x48415f48522f3530 | 12442 | HA_HR/50 |             4
 \x48415f48522f3530 | 14052 | HA_HR/50 |             4
(2 rows)

As a result it found 2 rows that is truly duplicates.

The unique index existed before the table was populated with data.
But how it can be possible with unique index described above?

Is this a bug? If needed, i can upload full binary copy of database
(~136Mb).