istio: endpoint /healthz/ready always show 200 response after /healthcheck/fail

Bug Description

Description We run the ingress-gateways with external loadbalancer in front of the nodes. The loadbalancer calls the /healthz/readyendpoint to check if the ingress-gateways are ready.

/healthz/readyendpoint always show 200 response, after call http://localhost:15000/healthcheck/fail

[step 1] Initially, it is in the LIVE state.
$ curl http://localhost:15000/ready 
LIVE
$ curl http://localhost:15021/healthz/ready -w '%{http_code}'
200

[step 2] call /healthcheck/fail and endpoint /ready show 503 DRAINING 
$ curl http://localhost:15000/healthcheck/fail -X POST
OK
$ curl http://localhost:15000/ready -w '%{http_code}'
DRAINING
503

[step 3] but /healthz/ready show 200 response
$ curl http://localhost:15021/healthz/ready -w '%{http_code}'
200
-> I hope to get 503 response, but I get 200 response

Expected behavior The http://localhost:15021/healthz/ready endpoint show 503 response after call http://localhost:15000/healthcheck/fail

Version

$ istioctl version
client version: 1.12.1
control plane version: 1.12.1
data plane version: 1.12.1 (7 proxies)

Additional Information

No response