This is resolved. I confirm that I was seeing the bug in CentOS 8 I indicated previously.
The system is on Linode, and a few weeks ago I opted to boot a Linode maintained version 5 kernel. That tripped the bug, in that recent versionos of systemctl have a dependency on the kernel that I have never seen before. Tonight I booted the system back to the C8 version 4 kernel, and now Virtualmin allows me to choose the version of php to run for a given vhost.