The upcoming rpm-4.19 automatically adds user/group requires for non-root owned
files.

The systemd and systemd-mini package contain a /var/log/journal directory that
has group "systemd-journal". They also contain a
/usr/lib/sysusers.d/systemd-journal.conf file that contains the group
definition.

We automatically create the user/group provides from the sysusers specification
if the sysuser-tools is installed at build time. But this is not the case for
systemd.

So, could you either add a "BuildRequires:  sysuser-tools" or add a "Provides:
group(systemd-journal)" to the systemd and systemd-mini packages.