Description of problem:
# oc -n openshift-monitoring logs -c prometheus prometheus-k8s-0 | grep 403
level=error ts=2020-05-18T06:58:54.532Z caller=notifier.go:524 component=notifier alertmanager=https://10.131.0.11:9095/api/v2/alerts count=1 msg="Error sending alert" err="bad response status 403 Forbidden"
level=error ts=2020-05-18T06:58:56.733Z caller=notifier.go:524 component=notifier alertmanager=https://10.131.0.11:9095/api/v2/alerts count=1 msg="Error sending alert" err="bad response status 403 Forbidden"
level=error ts=2020-05-18T08:28:54.537Z caller=notifier.go:524 component=notifier alertmanager=https://10.128.2.12:9095/api/v2/alerts count=1 msg="Error sending alert" err="bad response status 403 Forbidden"
...

# oc -n openshift-monitoring get pod -o wide | grep alertmanager
alertmanager-main-0                           5/5     Running   0          10h     10.129.2.5     ip-10-0-170-129.ap-south-1.compute.internal   <none>           <none>
alertmanager-main-1                           5/5     Running   0          10h     10.131.0.11    ip-10-0-141-41.ap-south-1.compute.internal    <none>           <none>
alertmanager-main-2                           5/5     Running   0          10h     10.128.2.12    ip-10-0-159-243.ap-south-1.compute.internal   <none>           <none>

it seems the alertmanager works fine
# token=`oc sa get-token prometheus-k8s -n openshift-monitoring`
# oc -n openshift-monitoring exec -c alertmanager alertmanager-main-1 -- curl -k -H "Authorization: Bearer $token" https://10.131.0.11:9095/api/v2/alerts | jq | tail
      "job": "kube-state-metrics",
      "job_name": "image-pruner-1589760000",
      "namespace": "openshift-image-registry",
      "pod": "kube-state-metrics-5dfb57cddc-mq4n8",
      "prometheus": "openshift-monitoring/k8s",
      "service": "kube-state-metrics",
      "severity": "warning"
    }
  }
]

# oc -n openshift-monitoring exec -c alertmanager alertmanager-main-1 -- curl -k -H "Authorization: Bearer $token" https://10.128.2.12:9095/api/v2/alerts | jq | tail
      "job": "kube-state-metrics",
      "job_name": "image-pruner-1589760000",
      "namespace": "openshift-image-registry",
      "pod": "kube-state-metrics-5dfb57cddc-mq4n8",
      "prometheus": "openshift-monitoring/k8s",
      "service": "kube-state-metrics",
      "severity": "warning"
    }
  }
]


Version-Release number of selected component (if applicable):
4.5.0-0.nightly-2020-05-17-201019

How reproducible:
In recent build

Steps to Reproduce:
1. See the description
2.
3.

Actual results:
"bad response status 403 Forbidden" in prometheus container logs

Expected results:
no error

Additional info: