helm-diff: Failure to honor lookup function

Unpack diffdemo.zip whose interesting part is

apiVersion: v1
kind: Secret
metadata:
  name: x
type: Opaque
data:
{{- $existing := (lookup "v1" "Secret" .Release.Namespace "x" ) }}
{{- if $existing }}
  text: {{ index $existing.data "text" }}
{{- else }}
  text: {{ (randAlpha 10) | b64enc}}
{{- end }}

and then run

$ helm install test .
NAME: test
LAST DEPLOYED: Mon Mar  1 09:13:24 2021
NAMESPACE: difftest
STATUS: deployed
REVISION: 1
TEST SUITE: None
NOTES:
kubectl get secret --namespace difftest x -o go-template="{{.data.text | base64decode}}{{\"\n\"}}"
$ kubectl get secret --namespace difftest x -o go-template="{{.data.text | base64decode}}{{\"\n\"}}"
eLbQJJESrO
$ helm upgrade test .
Release "test" has been upgraded. Happy Helming!
NAME: test
LAST DEPLOYED: Mon Mar  1 09:13:37 2021
NAMESPACE: difftest
STATUS: deployed
REVISION: 2
TEST SUITE: None
NOTES:
kubectl get secret --namespace difftest x -o go-template="{{.data.text | base64decode}}{{\"\n\"}}"
$ kubectl get secret --namespace difftest x -o go-template="{{.data.text | base64decode}}{{\"\n\"}}"
eLbQJJESrO
$ helm diff upgrade test .
difftest, x, Secret (v1) has changed:
  # Source: diffdemo/templates/secret.yaml
  apiVersion: v1
  kind: Secret
  metadata:
    name: x
  data:
-   text: '-------- # (10 bytes)'
+   text: '++++++++ # (10 bytes)'
  type: Opaque

helm diff upgrade claims that secret/x would change if helm upgrade were run, when in fact it would not. See #176 and https://github.com/helm/charts/issues/5167#issuecomment-641558251.

(Helm 3.5.2, Microk8s 1.20.2, kubectl 1.20.4, helm-diff freshly installed so I suppose 3.1.3.)