BUG #18625: user-created extensions change ownership to "postgres" after upgrade

The following bug has been logged on the website:

Bug reference:      18625
Logged by:          Evgeni Golov
Email address:      evgeni@golov.de
PostgreSQL version: 16.4
Operating system:   CentOS Stream 9, Debian 12
Description:

Hi,

when upgrading 15 to 16 (but also observed the same on 12 to 13), an
extension that was previously created by a "normal" user is now owned by
"postgres":

psql (15.8 (Debian 15.8-0+deb12u1))
SSL connection (protocol: TLSv1.3, cipher: TLS_AES_256_GCM_SHA384,
compression: off)
Type "help" for help.

foreman=> create extension cube;
CREATE EXTENSION
foreman=> select * from pg_extension;
  oid  | extname | extowner | extnamespace | extrelocatable | extversion |
extconfig | extcondition 
-------+---------+----------+--------------+----------------+------------+-----------+--------------
 13559 | plpgsql |       10 |           11 | f              | 1.0        |
        | 
 16386 | cube    |    16384 |         2200 | t              | 1.5        |
        | 

<upgrade to 16.4 via apt.postgresql.org>

foreman=> select * from pg_extension;
  oid  | extname | extowner | extnamespace | extrelocatable | extversion |
extconfig | extcondition 
-------+---------+----------+--------------+----------------+------------+-----------+--------------
 13562 | plpgsql |       10 |           11 | f              | 1.0        |
        | 
 16386 | cube    |       10 |         2200 | t              | 1.5        |
        | 
(2 rows)

Interestingly, the entry in pg_shdepend is still owned by the user.
 
foreman=> select * from pg_shdepend;
 dbid | classid | objid | objsubid | refclassid | refobjid | deptype 
------+---------+-------+----------+------------+----------+---------
    0 |    1262 | 16385 |        0 |       1260 |    16384 | o
(1 row)

I am reading https://postgrespro.com/list/thread-id/2333512 as "those two
should match".