Google Cloud auto-scaling thrashes between 0 and 1, even with minimum of 1

I have a managed instance group with auto scaling enabled. I have a minimum of 1 and maximum of 10 with health checks and cpu 0.8

The number of instances continually switches between 0 and 1. Every few minutes. I am unable to find the reason GCP decides to remove an instance and then immediately add it back. Health checks have no logs anywhere.

More concerning is that the minimum instances required is violated.

Thoughts? Thanks!

Edit: This may be due to instances becoming unhealthy, most likely because a firewall rule was needed to allow health checks on the instances. The health check worked for load balancing, but not instance health it seems. I'm using a custom network, so I needed to add the firewall rule.

https://cloud.google.com/compute/docs/load-balancing/health-checks#configure_a_firewall_rule_to_allow_health_checking

Will confirm/update after some monitoring time.