(GCP support here) This is a known bug, I've worked at least a few cases where this happened. There is a feature coming out soon that will allow different maintenance schedules to be set for masters/replicas, which will likely be automatically set for different times. And, once the kinks get worked out, hopefully we'll be able to re-deploy the feature that shifted traffic to failovers while the master is being updated, and eliminating maintenance downtime altogether.