If I run this command on a fresh installation:
  dscreate create-template temp ; dscreate from-file temp
the database is created in /dev/shm/slapd-localhost instead of
/var/lib/dirsrv/slapd-localhost/db.
running:
  dscreate create-template --adv /dev/stdout | grep 'db_.*='
shows indeed:
  ;db_dir = /var/lib/dirsrv/slapd-{instance_name}/db
  ;db_home_dir = /dev/shm/slapd-{instance_name}

This issue is currently present in leap 15.3,15.4 and tumbleweed.

Since I know by past tests that in leap 15.3 the issue was not present, I went
back and found that the problem was introduced in
389-ds-1.4.4.19~git28.b12c72226 while 389-ds-1.4.4.17~git0.5e1e392ae was fine.

I guess no one has ever created a fresh ds instance in leap 15.4 otherwise
somebody else should have reported this problem earlier.

Of course the workaround is to set db_home_dir the same as db_dir in the
template file used by "dscreate from-file"