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`.
return nextValue;
}
function mountDeferredValue<T>(value: T): T {
  const hook = mountWorkInProgressHook();
  hook.memoizedState = value;
  return value;
}
function updateDeferredValue<T>(value: T): T {
  const hook = updateWorkInProgressHook();
  const resolvedCurrentHook: Hook = (currentHook: any);
  const prevValue: T = resolvedCurrentHook.memoizedState;
  return updateDeferredValueImpl(hook, prevValue, value);
}
function rerenderDeferredValue<T>(value: T): T {
  const hook = updateWorkInProgressHook();
  if (currentHook === null) {
    
    hook.memoizedState = value;
    return value;
  } else {
    
    const prevValue: T = currentHook.memoizedState;
    return updateDeferredValueImpl(hook, prevValue, value);
  }
}
function updateDeferredValueImpl<T>(hook: Hook, prevValue: T, value: T): T {
  const shouldDeferValue = !includesOnlyNonUrgentLanes(renderLanes);
  if (shouldDeferValue) {