The prometheus operator fails to reconcile when proxy settings like no_proxy are set in the Alertmanager configuration secret.    

4.15.z and later    

    Always when AlertmanagerConfig is enabled

    1. Enable UWM with AlertmanagerConfig
    enableUserWorkload: true
    alertmanagerMain:
      enableUserAlertmanagerConfig: true
    2. Edit the "alertmanager.yaml" key in the alertmanager-main secret (see attached configuration file)
    3. Wait for a couple of minutes.
    

Monitoring ClusterOperator goes Degraded=True.
    

No error
    

The Prometheus operator logs show that it doesn't understand the proxy_from_environment field.
The newer proxy fields are supported since Alertmanager v0.26.0 which is equivalent to OCP 4.15 and above.