Yeah, my thought is not specific to this scenario. Indeed multi-AZ is a low cost and probably good idea because you often have a shared service management, control plane, and cheap bandwidth between things. Of course, when things fail they often ripple as may be the case here. I don't think clouds have their blast radius perfectly contained and they certainly don't communicate those details well.

One incident I recall was involving our GCP regional storage buckets, which we were using to achieve mutli-region redundancy. One day, both regions went down simultaneously. Google told us that the data was safe but the control plane and API for the service is global. Now I always wonder when I read about MR what that actually means...