fix: preserve nested arrays when cloning omitted json field data

packages/payload/src/fields/hooks/beforeChange/cloneDataFromOriginalDoc.ts

@@ -5,6 +5,12 @@ export const cloneDataFromOriginalDoc = (
 ): JsonArray | JsonObject => {
   if (Array.isArray(originalDocData)) {
     return originalDocData.map((row) => {
+      // Recurse for nested arrays so they stay arrays instead of being
+      // spread into index-keyed objects (`{...[1, 2]}` -> `{ 0: 1, 1: 2 }`).
+      if (Array.isArray(row)) {
+        return cloneDataFromOriginalDoc(row)
+      }
+
       if (typeof row === 'object' && row != null) {
         return {
           ...row,

test/fields/int.spec.ts

@@ -4318,6 +4318,36 @@ describe('Fields', () => {
       expect(updatedJsonFieldsDoc.json.state).toEqual({})
     })
 
+    it('should preserve nested arrays in a json field omitted from a partial update', async () => {
+      const arrayOfArrays = [
+        [1, 2],
+        [3, 4],
+      ]
+
+      const jsonFieldsDoc = await payload.create({
+        collection: 'json-fields',
+        data: {
+          customJSON: arrayOfArrays,
+        },
+      })
+
+      expect(jsonFieldsDoc.customJSON).toStrictEqual(arrayOfArrays)
+
+      // Partial update that intentionally omits `customJSON` so that its
+      // existing value is cloned from the original doc via
+      // `cloneDataFromOriginalDoc`. Nested arrays must survive that clone
+      // instead of being turned into index-keyed objects.
+      const updatedJsonFieldsDoc = await payload.update({
+        id: jsonFieldsDoc.id,
+        collection: 'json-fields',
+        data: {
+          json: { foo: 'bar' },
+        },
+      })
+
+      expect(updatedJsonFieldsDoc.customJSON).toStrictEqual(arrayOfArrays)
+    })
+
     describe('querying', () => {
       let fooBar
       let bazBar