ServerSideDiff fails to detect spec changes on custom resource

[llavaud]

Describe the bug

Argo CD’s server-side diff does not detect changes to the spec of a custom resource (KServe InferenceService).
A manual sync correctly applies the updates.
If I switch to client-side diff using the annotation argocd.argoproj.io/compare-options: ServerSideDiff=false, Argo CD detects the changes and applies them as expected.

To Reproduce

1. Apply a custom resource like the following:

apiVersion: serving.kserve.io/v1beta1
kind: InferenceService
metadata:
  name: lla-test
spec:
  predictor:
    maxReplicas: 4
    minReplicas: 1
    model:
      env:
        - name: MODEL_VERSION
          value: "14"
      modelFormat:
        name: mlflow
      protocolVersion: v2

2. Commit a change to any field in spec (for example, change predictor.maxReplicas from 4 to 2).
3. With server-side diff enabled (argocd.argoproj.io/compare-options: ServerSideDiff=true), Argo CD shows the application as Synced and Healthy, and does not detect the spec difference.
4. Switch to client-side diff (argocd.argoproj.io/compare-options: ServerSideDiff=false), Hit refresh, the difference is then detected and applied.

Observed behavior

• With server-side diff: No differences are detected; the app remains Synced/Healthy even when spec fields have changed; changes on metadata are detected and applied correctly.
• With client-side diff: Differences are detected and applied correctly.
• Manual sync: Updates the resource correctly.

Expected behavior

Argo CD should detect differences between live and desired manifests for this resource when using server-side diff, just like it does with client-side diff.

Screenshots

• Desired state

• Live state

• Application status

Application spec

apiVersion: argoproj.io/v1alpha1
kind: Application
metadata:
  annotations:
    argocd.argoproj.io/compare-options: ServerSideDiff=true
  finalizers:
  - resources-finalizer.argocd.argoproj.io
  name: test-app-test2
  namespace: argo-cd
spec:
  destination:
    name: gke-alpha-01-europe-west1
    namespace: test-app
  project: sre
  source:
    helm:
      releaseName: foo
    path: chart
    repoURL: https://github.com/REDACTED/manifests-test-app.git
    targetRevision: master
  syncPolicy:
    automated:
      prune: true
      selfHeal: true
    syncOptions:
    - CreateNamespace=true
    - ServerSideApply=true

Version

argocd: v3.2.1+8c4ab63
  BuildDate: 2025-11-30T11:48:14Z
  GitCommit: 8c4ab63a9c72b31d96c6360514cda6254e7e6629
  GitTreeState: clean
  GoVersion: go1.25.0
  Compiler: gc
  Platform: linux/amd64

[llavaud]

It’s quite odd—this doesn’t happen with other resources (like Deployments) or even other Custom Resources. It seems specific to this particular “special” CR.

Here's the CRD in case it helps:

apiVersion: apiextensions.k8s.io/v1
kind: CustomResourceDefinition
metadata:
  annotations:
    controller-gen.kubebuilder.io/version: v0.16.2
  creationTimestamp: "2025-03-26T11:14:43Z"
  generation: 1
  name: inferenceservices.serving.kserve.io
  resourceVersion: "565094707"
  uid: c1081997-d814-46b3-ac3d-16441cdd0ac7
spec:
  conversion:
    strategy: None
  group: serving.kserve.io
  names:
    kind: InferenceService
    listKind: InferenceServiceList
    plural: inferenceservices
    shortNames:
    - isvc
    singular: inferenceservice
  scope: Namespaced
  versions:
  - additionalPrinterColumns:
    - jsonPath: .status.url
      name: URL
      type: string
    - jsonPath: .status.conditions[?(@.type=='Ready')].status
      name: Ready
      type: string
    - jsonPath: .status.components.predictor.traffic[?(@.tag=='prev')].percent
      name: Prev
      type: integer
    - jsonPath: .status.components.predictor.traffic[?(@.latestRevision==true)].percent
      name: Latest
      type: integer
    - jsonPath: .status.components.predictor.traffic[?(@.tag=='prev')].revisionName
      name: PrevRolledoutRevision
      type: string
    - jsonPath: .status.components.predictor.traffic[?(@.latestRevision==true)].revisionName
      name: LatestReadyRevision
      type: string
    - jsonPath: .metadata.creationTimestamp
      name: Age
      type: date
    name: v1beta1
    schema:
      openAPIV3Schema:
        properties:
          apiVersion:
            type: string
          kind:
            type: string
          metadata:
            type: object
          spec:
            type: object
            x-kubernetes-map-type: atomic
            x-kubernetes-preserve-unknown-fields: true
          status:
            type: object
            x-kubernetes-map-type: atomic
            x-kubernetes-preserve-unknown-fields: true
        type: object
    served: true
    storage: true
    subresources:
      status: {}