Error when using encryptionSecret and prometheus alertManager Template

What happened:
When we tried to deploy kube-prometheus-stack by using PipeCD, plan preview failed like this↓

What you expected to happen:

We want to succeed to do a plan preview.

How to reproduce it:

• Register k8s app with a helm chart kube-prometheus-stack
• use both secret management (from PipeCD) and custom alert template in values.yaml like below↓

values.yaml

alertmanager:
...
  config:
    receivers:
    - name: 'pagerduty-notifications'
      pagerduty_configs:
      - service_key: {{.encryptedSecrets.key}}
  templateFiles:
     template_2.tmpl: |-
       {{ define "slack.custom.v2.text" }}
       {{ range .Alerts }}
         {{ if eq .Status "firing" }}{{ .Annotations.description }}{{else}}{{ .Annotations.resolved_description }}{{end}}
       {{ end }}
       {{ end }}

       {{ define "slack.custom.v2.title" }} [{{ .Status | toUpper }}{{ if eq .Status "firing" }}:{{ .Alerts.Firing | len }}{{ end }} {{ .GroupLabels.severity }}] {{ .GroupLabels.alertname }} {{ end }}

Environment:

• piped version:
• control-plane version:
• Others:

Reproduce the plan preview with a kube-prometheus-stack Helm chart, using both PipeCD secret management and the values.yaml alertmanager configuration shown in the issue. Start by isolating whether encryptionSecret or the custom alert template triggers the failure. Done means the same configuration completes plan preview successfully.