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`.
memoizedState: currentHook.memoizedState,
      baseState: currentHook.baseState,
      baseQueue: currentHook.baseQueue,
      queue: currentHook.queue,
      next: null,
    };
    if (workInProgressHook === null) {
      
      currentlyRenderingFiber.memoizedState = workInProgressHook = newHook;
    } else {
      
      workInProgressHook = workInProgressHook.next = newHook;
    }
  }
  return workInProgressHook;
}
let createFunctionComponentUpdateQueue: () => FunctionComponentUpdateQueue;
if (enableUseMemoCacheHook) {
  createFunctionComponentUpdateQueue = () => {
    return {
      lastEffect: null,
      events: null,
      stores: null,
      memoCache: null,
    };
  };
} else {
  createFunctionComponentUpdateQueue = () => {
    return {
      lastEffect: null,