Description of problem: 

When a SNO is detatched gracefully it is expected that all the alerts for that SNO are cleared and resolved permanently. But however it is seen that hours after the SNO is detatched the alerts for that SNO are still active and firing.

 
Removal of ACM does not remove the Prometheus configuration on that cluster - this can be confirmed by looking at cluster-monitoring-config configmap in the managed cluster. So it still keeps on sending alerts to the alert manager.
 
apiVersion: v1
data:
config.yaml: |
alertmanagerMain: null
enableUserWorkload: null
grafana: null
http: null
k8sPrometheusAdapter: null
kubeStateMetrics: null
openshiftStateMetrics: null
prometheusK8s:
additionalAlertManagerConfigs:

• apiVersion: v2
  bearerToken:
  key: token
  name: observability-alertmanager-accessor
  pathPrefix: /
  scheme: https
  staticConfigs:
• alertmanager-open-cluster-management-observability.apps.rchltx-acm-4.faredge.vzwops.com         -
  tlsConfig:
  ServerName: ""
  ca:
  key: service-ca.crt
  name: hub-alertmanager-router-ca
  insecureSkipVerify: false
  externalLabels:
  managed_cluster: c57064dd-519b-4678-a920-93d034a3e645
  logLevel: ""
  nodeSelector: null
  remoteWrite: null
  resources: null
  retention: ""
  tolerations: null
  volumeClaimTemplate: null
  prometheusOperator: null
  telemeterClient: null
  thanosQuerier: null
  kind: ConfigMap
  metadata:
  creationTimestamp: "2023-03-16T05:05:48Z"
  name: cluster-monitoring-config
  namespace: openshift-monitoring
  resourceVersion: "63358"
  uid: 6f974671-a306-43ea-9be3-ade8b7712236