Found the source of this issue and a solid workaround. Apparently Virtuozzo/OpenVZ symlinks /sbin/init to systemd within the container, but then on the node when vzctl launches things it passes init parameters in like: init -z, which results in systemd -z.
For reasons unknown to me, the -z seriously messes with systemd particularly after systemctl reexec-daemon is run, which is exactly what Plesk does when it runs system updates.
There is a patch available for systemd to prevent the unexpected switch from messing with systemd upon reexec, but RedHat hasn't yet backported it. Virtuozzo devs haven't yet taken note, but the original reporter of the issue on the systemd project on Github devised a solid workaround to replace the /sbin/init symlink with a quick script to strip the -z from the parameters that vzctl feeds into init, resulting in systemd never having the -z paramter passed into it.