When trying to send the alerting emails with a text template instead of html, the alertmanager would not use the text template and send html emails. once we set alertmanagerMain:
      enableUserAlertmanagerConfig: false
then the emails were showing up with the text template we specified. 

openshift cluster: 4.10.25

very reproducible. reproduced this error on an openshift-demo 4.10 cluster

1. following this documentation to set up openshift alert manager: https://docs.openshift.com/container-platform/4.10/monitoring/managing-alerts.html
2. confirm that alertmanager receiver email configuration looks like:
email_configs:
      - to: 'someone@email.com'
        from: something@email.com
        smarthost: 'email:25'
        require_tls: false
        send_resolved: true
        html: ''
        text: |-
          {{ range .Alerts }}
          *Alert:* {{ .Annotations.summary }} - `{{ .Labels.severity }}`
          *Description:* {{ .Annotations.description }}
          *Details:*
          {{ range .Labels.SortedPairs }} * *{{ .Name }}:* `{{ .Value }}`
          {{ end }}
          {{ end }}
3. cluster-monitoring-config Configmap in the openshift-monitoring has the following at the top:
data:
  config.yaml: |
    enableUserWorkload: true
    alertmanagerMain:
      enableUserAlertmanagerConfig: true
      nodeSelector:
        node-role.kubernetes.io/infra: ""
      tolerations:
      - key: node-role.kubernetes.io/infra
        operator: Exists
        effect: NoSchedule
      volumeClaimTemplate:
        spec:
          storageClassName: ocs-storagecluster-ceph-rbd
          resources:
            requests:
              storage: 40Gi
  4. trigger an email alert: 
oc exec alertmanager-main-0 -- amtool alert add --alertmanager.url http://localhost:9093 alertname=justchecking --start="2023-04-03T00:01:18-00:00" severity=info --annotation=summary="summary" --annotation=description="alert desc."

5. check email. The email template is in the default html instead of the defined text template. 

6. change "alertmanagerMain: enableUserAlertmanagerConfig: false" in the ecluster-monitoring-config Configmap and re-trigger the email alert, the email template should now be in the specified text template from the email configs. 

html template

text template

I can provide screenshots and upload additional configuration files if allowed