App Engine flexible restarting from 503 /readiness_check failure

Question

My app.yaml configuration has a custom URL /my_readiness_check for readiness check instead of the default /readiness_check.

I see requests to the custom URL that succeed. Then I see my instance restarted with a SIGKILL to the app and they are immediately preceded by a couple of 503 failures to /readiness_check

runtime: python
env: flex

readiness_check:
  path: "/my_readiness_check"
  check_interval_sec: 10
  timeout_sec: 4
  failure_threshold: 1
  success_threshold: 2
  app_start_timeout_sec: 300

liveness_check:
  path: "/my_liveness_check"
  check_interval_sec: 30
  timeout_sec: 30
  failure_threshold: 3
  initial_delay_sec: 60

...