Hope our "DOH!" moment helps others.
As a result of Article ID: 369767, vMotion fails with error "NamespaceMgr could not lock the db file," we updated our Image Mappings' templates' to hardware version to 21 and then performed the required Cloud Account synchs.
Subsequent deployments failed with the following error: "Provisioning operation failed. Error from vCenter: Error caused by file /vmfs/volumes/vsan:<UID>/<template.vmdk>." Internet searches implied a corrupt vmdk; however, in vSphere we could deploy new VMs from it.
Assuming we had a bad template, we decided to recreate it. When we tried to delete it, the cause was revealed - "Invalid virtual machine state. Unable to delete template with snapshots - convert the template to a Virtual Machine first." As Article ID: 315390, "Upgrading a virtual machine to the latest hardware version (multiple versions)," clearly states, a snapshot is automatically created by the VM hardware upgrade process.