Hmm. I've asked the authoritative DNS team to explain what's happening here. I'll let HN know when I get an authoritative answer. It's been a few years since I looked at the code and a whole bunch of people keep changing it :-)

My suspicion is that this is to do with the fact that we want to keep affinity between the client IP and a backend server (which OP mentions in their blog). And the question is "do you break that affinity if the backend server goes down?" But I'll reply to my own comment when I know more.