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 check: StoreConsistencyCheck<T> = {
    getSnapshot,
    value: renderedSnapshot,
  };
  let componentUpdateQueue: null | FunctionComponentUpdateQueue =
    (currentlyRenderingFiber.updateQueue: any);
  if (componentUpdateQueue === null) {
    componentUpdateQueue = createFunctionComponentUpdateQueue();
    currentlyRenderingFiber.updateQueue = (componentUpdateQueue: any);
    componentUpdateQueue.stores = [check];
  } else {
    const stores = componentUpdateQueue.stores;
    if (stores === null) {
      componentUpdateQueue.stores = [check];
    } else {
      stores.push(check);
    }
  }
}
function updateStoreInstance<T>(
  fiber: Fiber,
  inst: StoreInstance<T>,
  nextSnapshot: T,
  getSnapshot: () => T,
): void {
  
  inst.value = nextSnapshot;
  inst.getSnapshot = getSnapshot;