Permission for "/var/log/journal/'machine-id'/*"

I want to change the permission of the log file stored in /var/log/journal/'machine-id'/* to 640.

After changing "/usr/lib/tmpfiles.d/systemd.conf", I manually changed the permission, but it reverts after a while.
I then changed the permissions manually, but after some time it reverted.

-rw-r-x---+ 1 root systemd-journal 8388608 Jun 20 06:39 user-1000.journal

I would appreciate it if you could give me a way to permanently set "/var/log/journal/'machine-id'/*" permissions to 640.