BUG #17938: could not open shared memory segment "/PostgreSQL.615216676": No such file or directory

The following bug has been logged on the website:

Bug reference:      17938
Logged by:          Onkobu Tanaake
Email address:      onkobu@onkobutanaake.de
PostgreSQL version: 15.3
Operating system:   Gentoo
Description:

After upgrade from 14 to 15 from time to time this error occurs. A snapshot
of a responsive database looks like this:

$> ls -la /dev/shm/PostgreSQL.*
-rw------- 1 postgres postgres 1048576 May 22 19:18
/dev/shm/PostgreSQL.2644390168
-rw------- 1 postgres postgres   26176 May 22 19:14
/dev/shm/PostgreSQL.3338605124

$> lsof | grep PostgreSQL
postgres  16944                   postgres  mem       REG               0,49
    26176   
       2 /dev/shm/PostgreSQL.3338605124
postgres  16945                   postgres  mem       REG               0,49
    26176   
       2 /dev/shm/PostgreSQL.3338605124
postgres  16946                   postgres  mem       REG               0,49
    26176   
       2 /dev/shm/PostgreSQL.3338605124
postgres  16948                   postgres  mem       REG               0,49
    26176   
       2 /dev/shm/PostgreSQL.3338605124
postgres  16949                   postgres  mem       REG               0,49
  1048576   
       3 /dev/shm/PostgreSQL.2644390168
postgres  16949                   postgres  mem       REG               0,49
    26176   
       2 /dev/shm/PostgreSQL.3338605124
postgres  16950                   postgres  mem       REG               0,49
  1048576   
       3 /dev/shm/PostgreSQL.2644390168
postgres  16950                   postgres  mem       REG               0,49
    26176   
       2 /dev/shm/PostgreSQL.3338605124
postgres  16958                   postgres  mem       REG               0,49
  1048576   
       3 /dev/shm/PostgreSQL.2644390168
postgres  16958                   postgres  mem       REG               0,49
    26176   
       2 /dev/shm/PostgreSQL.3338605124

$> df -h
Filesystem             Size  Used Avail Use% Mounted on
[…]
shm                     32G  1.1M   32G   1% /dev/shm
[…]

Config (postgresql.conf) is as follows:

max_wal_size = 2GB
min_wal_size = 1GB
effective_cache_size = 8GB
log_timezone = 'Europe/Berlin'
datestyle = 'iso, mdy'
timezone = 'Europe/Berlin'
default_text_search_config = 'pg_catalog.english'
plperl.on_init = 'use utf8; use re; package utf8; require
"utf8_heavy.pl";'

I don't use shared_memory_type and also not dynamic_shared_memory_type so
they're default (mmap for the first, posix for the second). Got the settings
from https://pgtune.leopard.in.ua/ for a conservative use on a 32GByte 8
core/ 16 threads machine.

Building goes along the lines of an ebuild
https://gitweb.gentoo.org/repo/gentoo.git/tree/dev-db/postgresql/postgresql-15.3.ebuild#n117.
I enabled icu, kerberos, nls, pam, ssl, threads, uuid, xml and zlib.

The server does not run any systemd but is pure Init-V.  So it is very
unlikely that a systemd process causes this.Also no container in use. Init
process directly runs postgres server. Kernel is 5.15.80 and was the same
for version 14. I also don't log in with the postgres-user which is also
impossible (disabled in /etc/shadow). WIthin 7 days this happens very
likely/ at least once. How can I track this down further?