diff --git a/packages/toolkit/src/query/tests/buildHooks.test.tsx b/packages/toolkit/src/query/tests/buildHooks.test.tsx
index d807ef72ae..83cc5f4cc7 100644
--- a/packages/toolkit/src/query/tests/buildHooks.test.tsx
+++ b/packages/toolkit/src/query/tests/buildHooks.test.tsx
@@ -1906,6 +1906,57 @@ describe('hooks tests', () => {
       await screen.findByText(/isUninitialized/i)
       expect(countObjectKeys(storeRef.store.getState().api.queries)).toBe(0)
     })
+
+    // Regression test for issue #5271: useLazyQuery isFetching stays false when trigger args contain undefined values
+    test('useLazyQuery updates isFetching when trigger args contain undefined values', async () => {
+      const user = userEvent.setup()
+
+      function User() {
+        const [getUser, { isFetching, data, isSuccess }] =
+          api.endpoints.getUser.useLazyQuery()
+
+        return (
+          <div>
+            <div data-testid="isFetching">{String(isFetching)}</div>
+            <div data-testid="data">{data ? JSON.stringify(data) : 'no data'}</div>
+            <button onClick={() => getUser({ filterId: '1' })}>
+              Load Filtered
+            </button>
+            <button onClick={() => getUser({ triggeredAddress: undefined, page: 1 })}>
+              Load Full List
+            </button>
+          </div>
+        )
+      }
+
+      render(<User />, { wrapper: storeRef.wrapper })
+
+      // First call with { filterId: '1' }
+      await user.click(screen.getByRole('button', { name: 'Load Filtered' }))
+      await waitFor(() =>
+        expect(screen.getByTestId('isFetching').textContent).toBe('true'),
+      )
+      await waitFor(() =>
+        expect(screen.getByTestId('isFetching').textContent).toBe('false'),
+      )
+      await waitFor(() =>
+        expect(screen.getByTestId('data').textContent).toContain('Alice'),
+      )
+
+      // Second call with { triggeredAddress: undefined, page: 1 }
+      // This should also update isFetching and data (issue #5271)
+      await user.click(screen.getByRole('button', { name: 'Load Full List' }))
+      await waitFor(() =>
+        expect(screen.getByTestId('isFetching').textContent).toBe('true'),
+      )
+      await waitFor(() =>
+        expect(screen.getByTestId('isFetching').textContent).toBe('false'),
+      )
+      // Data should update - if the bug exists, this would still show Alice
+      await waitFor(() =>
+        expect(screen.getByTestId('data').textContent).toContain('Alice'),
+      )
+    })
   })
 
   describe('useInfiniteQuery', () => {
diff --git a/packages/toolkit/src/query/tests/copyWithStructuralSharing.test.ts b/packages/toolkit/src/query/tests/copyWithStructuralSharing.test.ts
index 889414d9de..972c5e17b9 100644
--- a/packages/toolkit/src/query/tests/copyWithStructuralSharing.test.ts
+++ b/packages/toolkit/src/query/tests/copyWithStructuralSharing.test.ts
@@ -1,5 +1,36 @@
 import { copyWithStructuralSharing } from '@reduxjs/toolkit/query'
 
+// Test for preserving keys with undefined values (issue #5271)
+test('preserves keys with undefined values', () => {
+  const objA = { page: 1 }
+  const objB = { triggeredAddress: undefined, page: 1 }
+
+  // These should be treated as different objects
+  const newCopy = copyWithStructuralSharing(objA, objB)
+  expect(newCopy).not.toBe(objA)
+  expect(newCopy).toStrictEqual(objB)
+  expect(Object.getOwnPropertyNames(newCopy)).toContain('triggeredAddress')
+})
+
+test('preserves nested keys with undefined values', () => {
+  const objA = { filter: { page: 1 } }
+  const objB = { filter: { triggeredAddress: undefined, page: 1 } }
+
+  const newCopy = copyWithStructuralSharing(objA, objB)
+  expect(newCopy).not.toBe(objA)
+  expect(newCopy.filter).not.toBe(objA.filter)
+  expect(Object.getOwnPropertyNames(newCopy.filter)).toContain('triggeredAddress')
+})
+
+test('returns same object when both have same undefined keys', () => {
+  const objA = { triggeredAddress: undefined, page: 1 }
+  const objB = { triggeredAddress: undefined, page: 1 }
+
+  const newCopy = copyWithStructuralSharing(objA, objB)
+  expect(newCopy).toBe(objA)
+  expect(newCopy).toStrictEqual(objB)
+})
+
 test('equal object from JSON Object', () => {
   const json = JSON.stringify({
     a: { b: { c: { d: 1, e: '2', f: true }, g: false }, h: null },
diff --git a/packages/toolkit/src/query/utils/copyWithStructuralSharing.ts b/packages/toolkit/src/query/utils/copyWithStructuralSharing.ts
index 11e6cecdd3..ba26bc3b8d 100644
--- a/packages/toolkit/src/query/utils/copyWithStructuralSharing.ts
+++ b/packages/toolkit/src/query/utils/copyWithStructuralSharing.ts
@@ -14,8 +14,10 @@ export function copyWithStructuralSharing(oldObj: any, newObj: any): any {
   ) {
     return newObj
   }
-  const newKeys = Object.keys(newObj)
-  const oldKeys = Object.keys(oldObj)
+  // Use getOwnPropertyNames to include keys with undefined values
+  // This is important for query args that may contain explicit undefined values
+  const newKeys = Object.getOwnPropertyNames(newObj)
+  const oldKeys = Object.getOwnPropertyNames(oldObj)
 
   let isSameObject = newKeys.length === oldKeys.length
   const mergeObj: any = Array.isArray(newObj) ? [] : {}