We had a 2 node NLB cluster running IIS websites on virtual machines. Both nodes were online, the balancer functioned exactly as expected, (if traffic is 50/50 balanced, and you stop or drainstop a node, all traffic routes seamlessles to the other node.)
But when I rebooted a node, even if I stopped it prior to reboot, the OTHER node which should be receiving production traffic during the reboot stopped accepting requests.
To my knowledge this was NOT how NLB is supposed to work. If I power down a node, the other nodes in the NLB cluster shouldn't care, and should continue to accept traffic according to their port rules while the offline node reboots.
None of my port rules employed affinity, so I knew that wasn't the issue.
So after agonizing over it a bit, I stumbled on the answer (see my posted answer)