RouterCertsDegraded with x509 certificate signed by unknown authority in OpenShift 4

Issue

• After replacing the default ingress certificate with a custom one, the authentication Cluster Operator becomes degraded with the error below:

  RouterCertsDegraded: secret/v4-0-config-system-router-certs.spec.data[apps.example.com] -n openshift-authentication: certificate could not validate route hostname oauth-openshift.apps.example.com: x509: certificate signed by unknown authority
  

  Event(v1.ObjectReference{Kind:"Deployment", Namespace:"openshift-authentication-operator", Name:"authentication-operator", UID:"<UID>",                                                             
  APIVersion:"apps/v1", ResourceVersion:"", FieldPath:""}): type: 'Normal' reason: 'OperatorStatusChanged' Status for clusteroperator/authentication changed: Degraded changed from False to True ("RouterCertsDegraded: secret/v4-0-config-system-router-certs.spec.data[apps.example.com] -n openshift-authentication: certificate could not validate route hostname oauth-openshift.apps.example.com: x509: certificate signed by unknown authority")
  [...]