When the Alertmanager configuration secret holds additional keys and the Alertmanager configuration references them as templates with a relative path, receivers using the templates will fail to deliver notifications. 

4.12 only (not present in previous versions)

Only when using additional keys in the Alertmanager config secret + loading them as templates.

1. Update openshift-monitoring/alertmanager-main secret (see attached file).
2. Watch the alertmanager-main-0/alertmanager logs and wait for 5 minutes
3.

Notification error logs + AlertmanagerClusterFailedToSendAlerts alert pending

No error and no alert

Regression introduced by https://github.com/prometheus-operator/prometheus-operator/pull/4906