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`.
const queue: UpdateQueue<S, A> = {
    pending: null,
    lanes: NoLanes,
    dispatch: null,
    lastRenderedReducer: reducer,
    lastRenderedState: (initialState: any),
  };
  hook.queue = queue;
  const dispatch: Dispatch<A> = (queue.dispatch = (dispatchReducerAction.bind(
    null,
    currentlyRenderingFiber,
    queue,
  ): any));
  return [hook.memoizedState, dispatch];
}
function updateReducer<S, I, A>(
  reducer: (S, A) => S,
  initialArg: I,
  init?: I => S,
): [S, Dispatch<A>] {
  const hook = updateWorkInProgressHook();
  const queue = hook.queue;
  if (queue === null) {
    throw new Error(
      'Should have a queue. This is likely a bug in React. Please file an issue.',
    );
  }
  queue.lastRenderedReducer = reducer;
  const current: Hook = (currentHook: any);