Nested `startTransition` call "takes over" and makes parent `startTransition` unable to track `isPending`.

Seb says it's a bug.

Repro:

https://codesandbox.io/s/pensive-breeze-rg70wn?file=/IndexPage.js:192-288

1. Click the button
2. `isPending` in `IndexPage.js` is `true`

However, `isPending` in `App.js` is `false`.

Expected: `isPending` in `App.js` is also `true`.
/**
 * Copyright (c) Meta Platforms, Inc. and affiliates.
 *
 * This source code is licensed under the MIT license found in the
 * LICENSE file in the root directory of this source tree.
 *
 * @flow
 */
import type {
  MutableSource,
  MutableSourceGetSnapshotFn,
  MutableSourceSubscribeFn,
  ReactContext,
  StartTransitionOptions,
  Usable,
  Thenable,
} from 'shared/ReactTypes';
import type {
  Fiber,
  FiberRoot,
  Dispatcher,
  HookType,
  MemoCache,
} from './ReactInternalTypes';
import type {Lanes, Lane} from './ReactFiberLane';
import type {HookFlags} from './ReactHookEffectTags';
import type {Flags} from './ReactFiberFlags';
import ReactSharedInternals from 'shared/ReactSharedInternals';
import {