Adding Karel to cc, as lastlog is from util-linux package . Setup package just owns /var/log/lastlog - probably for historic reasons. It doesn't even create the file, so any package which creates the file is actually responsible for the unclean rpm -V output.
Karel, any suspects? (For me either shadow-utils or systemd).

We have in util-linux.spec file:

%post           
# only for minimal buildroots without /var/log
[ -d /var/log ] || mkdir -p /var/log
touch /var/log/lastlog
chown root:root /var/log/lastlog
chmod 0644 /var/log/lastlog
# Fix the file context, do not use restorecon
if [ -x /usr/sbin/selinuxenabled ] && /usr/sbin/selinuxenabled; then
        SECXT=$( /usr/sbin/matchpathcon -n /var/log/lastlog 2> /dev/null )
        if [ -n "$SECXT" ]; then
                # Selinux enabled, but without policy? It's true for buildroots
                # without selinux stuff on host machine with enabled selinux.
                # We don't want to use any RPM dependence on selinux policy for
                # matchpathcon(2). SELinux policy should be optional.
                /usr/bin/chcon "$SECXT"  /var/log/lastlog >/dev/null 2>&1 || :
        fi      
fi 

%files
%ghost %attr(0644,root,root) %verify(not md5 size mtime) /var/log/lastlog

The question is where "root.utmp" comes from. login(1) is able to create the file 
on the fly, but it does not explain "utmp" group.

Anyway, would be better move the file from setup to util-linux?

Ah, I see it:

$ git grep lastlog
...
tmpfiles.d/var.conf.m4:f /var/log/lastlog 0664 root utmp -


... systemd upstream tree :-)

It seems we need to sync
   /usr/lib/tmpfiles.d/var.conf:f /var/log/lastlog 0664 root utmp -

with util-linux spec file.

CC: systemd guys. Any suggestion -- why we need "root.utmp" for lastlog?

It seems to be a custom on other distributions that utmp group can write to these files. See https://github.com/systemd/systemd/issues/6994

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://access.redhat.com/errata/RHBA-2019:3603