I noticed that on cloudvirt1015 (a stretch host using the openstack-mitaka-jessie component), systemd-sysv was flagged for downgrade (from 232-25+deb9u11 to 230-7+bpo8+2 from the mitaka component), the other installed systemd packages are not flagged for downgrade.
systemd should be removed from the component, having different systemd packages installed on a system will lead to all kind of subtle issues.