Comment 38 for bug 1748147

Tested, works.

Repro (Xenial):
# dpkg -l | grep systemd
ii systemd 229-4ubuntu21.2 amd64 system and service manager

/var# ll
drwxrwxr-x 8 root syslog 4096 Jul 9 06:25 log/ <--775

# apt install --reinstall systemd

/var# ll
drwxr-xr-x 8 root syslog 4096 Jul 9 06:25 log/ <-- 755

Bionic (Verified):
# dpkg -l | grep systemd
ii systemd 237-3ubuntu10.2 amd64 system and service manager

/var# ll
drwxrwxr-x 8 root syslog 4096 Jul 9 13:09 log/ <-- 775

# apt install --reinstall systemd
0 upgraded, 0 newly installed, 1 reinstalled, 0 to remove and 0 not upgraded.
Need to get 2895 kB of archives.
After this operation, 0 B of additional disk space will be used.
Get:1 http://archive.ubuntu.com/ubuntu bionic-proposed/main amd64 systemd amd64 237-3ubuntu10.2 [2895 kB]
...

/var# ll
drwxrwxr-x 8 root syslog 4096 Jul 9 13:09 log/ <-- 775