fix: regression on creationTimestamp with server-side diff (#25210)

Signed-off-by: Peter Jiang <peterjiang823@gmail.com>

gitops-engine/pkg/diff/diff.go

@@ -183,6 +183,9 @@ func serverSideDiff(config, live *unstructured.Unstructured, opts ...Option) (*D
 		}
 	}
 
+	// Remarshal predictedLive to ensure it receives the same normalization as live.
+	predictedLive = remarshal(predictedLive, o)
+
 	Normalize(predictedLive, opts...)
 	unstructured.RemoveNestedField(predictedLive.Object, "metadata", "managedFields")
 

gitops-engine/pkg/diff/diff_test.go

@@ -1607,6 +1607,68 @@ metadata:
 	assert.False(t, ok)
 }
 
+func TestRemarshalStatefulSetCreationTimestamp(t *testing.T) {
+	manifest := []byte(`
+apiVersion: apps/v1
+kind: StatefulSet
+metadata:
+  name: test-sts
+  creationTimestamp: "2025-11-06T19:35:31Z"
+spec:
+  serviceName: test
+  selector:
+    matchLabels:
+      app: test
+  template:
+    metadata:
+      creationTimestamp: null
+      labels:
+        app: test
+    spec:
+      containers:
+      - name: test
+        image: nginx
+  volumeClaimTemplates:
+  - metadata:
+      name: data
+      creationTimestamp: null
+    spec:
+      accessModes:
+      - ReadWriteOnce
+      resources:
+        requests:
+          storage: 1Gi
+`)
+	var un unstructured.Unstructured
+	require.NoError(t, yaml.Unmarshal(manifest, &un))
+
+	// Verify creationTimestamp exists in nested metadata before remarshal
+	spec := un.Object["spec"].(map[string]any)
+	templateMetadata := spec["template"].(map[string]any)["metadata"].(map[string]any)
+	_, ok := templateMetadata["creationTimestamp"]
+	assert.True(t, ok, "creationTimestamp should exist in template.metadata before remarshal")
+
+	volumeClaimTemplates := spec["volumeClaimTemplates"].([]any)
+	vctMetadata := volumeClaimTemplates[0].(map[string]any)["metadata"].(map[string]any)
+	_, ok = vctMetadata["creationTimestamp"]
+	assert.True(t, ok, "creationTimestamp should exist in volumeClaimTemplates[0].metadata before remarshal")
+
+	// Remarshal
+	newUn := remarshal(&un, applyOptions(diffOptionsForTest()))
+
+	// Verify creationTimestamp is removed from nested metadata after remarshal
+	// (top-level metadata.creationTimestamp is preserved as it's part of the resource identity)
+	spec = newUn.Object["spec"].(map[string]any)
+	templateMetadata = spec["template"].(map[string]any)["metadata"].(map[string]any)
+	_, ok = templateMetadata["creationTimestamp"]
+	assert.False(t, ok, "creationTimestamp should be removed from template.metadata after remarshal")
+
+	volumeClaimTemplates = spec["volumeClaimTemplates"].([]any)
+	vctMetadata = volumeClaimTemplates[0].(map[string]any)["metadata"].(map[string]any)
+	_, ok = vctMetadata["creationTimestamp"]
+	assert.False(t, ok, "creationTimestamp should be removed from volumeClaimTemplates[0].metadata after remarshal")
+}
+
 func TestRemarshalResources(t *testing.T) {
 	getRequests := func(un *unstructured.Unstructured) map[string]any {
 		return un.Object["spec"].(map[string]any)["containers"].([]any)[0].(map[string]any)["resources"].(map[string]any)["requests"].(map[string]any)