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 localId = localIdCounter++;
    if (localId > 0) {
      id += 'H' + localId.toString(32);
    }
    id += ':';
  } else {
    
    const globalClientId = globalClientIdCounter++;
    id = ':' + identifierPrefix + 'r' + globalClientId.toString(32) + ':';
  }
  hook.memoizedState = id;
  return id;
}
function updateId(): string {
  const hook = updateWorkInProgressHook();
  const id: string = hook.memoizedState;
  return id;
}
function mountRefresh(): any {
  const hook = mountWorkInProgressHook();
  const refresh = (hook.memoizedState = refreshCache.bind(
    null,
    currentlyRenderingFiber,
  ));
  return refresh;
}
function updateRefresh(): any {