I discovered the problem with our cluster setup. Actually, there were two problems. First, the DRAC cards had the MTU set to 1500. Problem is that I have the internal network configured to use an MTU of 9000 to improve throughput with the storage SAN (NFS). This explains the hit-or-miss with fencing of the nodes. Second problem, one of the physical nodes was still using an MTU of 1500. This is what caused the cluster instabilities after one of the nodes was shutdown.
I've configured everything for an MTU of 1500 for the moment and have been pleasantly surprised that the node is fenced (turned off) when I yank the network on it. Also, the HA VMs are migrated to one of the remaining active nodes. I haven't figured out what the criteria is for a node to receive a particular HA VM, but it does work. I should mention that the node is restarted on another node, not live migrated. This makes some sense as this would require all nodes to have live copies of the active state (memory) of the HA VM, which I presume isn't (yet) possible with QEMU.