I have deployed Prometheus using click-to-deploy - "https://github.com/GoogleCloudPlatform/click-to-deploy/blob/master/k8s/prometheus/manifest/alertmanager-statefulset.yaml" on GKE cluster

When I make changes to alert-manager config map and restart the alert-manager pods, I don't get any alerts in alert-manager.

apiVersion: v1
kind: ConfigMap
metadata:
  name: $APP_INSTANCE_NAME-alertmanager-config
  labels:
    app.kubernetes.io/name: $APP_INSTANCE_NAME
    app.kubernetes.io/component: alertmanager
data:
  alertmanager.yml: |
    global:
      smtp_smarthost: 'XXXX:25'
      smtp_from: 'XX@XX.com'
    templates:
    - '/etc/alertmanager/template/*.tmpl'
    route:
      group_by: ['alertname', 'cluster', 'service']
      group_wait: 30s
      group_interval: 5m
      repeat_interval: 3h
      receiver: team-X-mails

    inhibit_rules:
    - source_match:
        severity: 'critical'
      target_match:
        severity: 'warning'

      equal: ['alertname', 'cluster', 'service'] 
    receivers:
    - name: 'team-X-mails'
      email_configs:
      - to: 'YY@YY.com'

When I use curl command to trigger alert in alertmanager, I can see the alert and get email notification.

Prometheus configmap for alert

alerts.yaml: |-
    "groups":
    - "name": "general.rules"
      "rules":
      - "alert": "TargetDown"
        "annotations":
          "description": "{{ $value }}% of {{ $labels.job }} targets are down."
          "summary": "Targets are down"
        "expr": "100 * (count(up == 0) BY (job) / count(up) BY (job)) > 10"
        "for": "10m"
        "labels":
          "severity": "critical"
      - "alert": "DeadMansSwitch"
        "annotations":
          "description": "This is a DeadMansSwitch meant to ensure that the entire Alerting pipeline is functional."
          "summary": "Alerting DeadMansSwitch"
        "expr": "vector(1)"
        "labels":
          "severity": "none"

I can see alert getting fired in prometheus.

prometheus-alertmanager-0                        1/1     Running     0          67m
prometheus-deployer-7vl74                        0/1     Completed   0          4h17m
prometheus-grafana-0                             1/1     Running     0          4h17m
prometheus-kube-state-metrics-7674795b44-4z6zr   2/2     Running     0          4h17m
prometheus-node-exporter-7bdvj                   1/1     Running     0          4h17m
prometheus-prometheus-0                          1/1     Running     0          77m

and

prometheus-alertmanager            LoadBalancer   X.X.X.X    X.X.X.X   9093:31157/TCP      4h18m
prometheus-alertmanager-operated   ClusterIP      None             <none>           6783/TCP,9093/TCP   112m
prometheus-kube-state-metrics      ClusterIP      X.X.X.X    <none>           8080/TCP,8081/TCP   4h18m
prometheus-prometheus              LoadBalancer   X.X.X.X   X.X.X.X   9090:31257/TCP      4h18m