We have a blue green deployment system in place that we are using for quite a while. We have two backend services on the load balancer. One service is for test and another is for production. There are 2 different node pools (k8s) used behind those backend services. To deploy new version to production we simply change instance group on the production backend service. It did work for quite a while until June 2019. After switching instance group for short period of time (about 2-3 mins) backend service is not available and LB respond with 502 error.
I've also created an issue in GCP bug tracker that includes screenshots and steps to reproduce - https://issuetracker.google.com/issues/136020917