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 prevSubscribe = memoizedState.subscribe;
  const fiber = currentlyRenderingFiber;
  hook.memoizedState = ({
    refs,
    source,
    subscribe,
  }: MutableSourceMemoizedState<Source, Snapshot>);
   Sync the values needed by our subscription handler after each commit.
  dispatcher.useEffect(() => {
    refs.getSnapshot = getSnapshot;
    
    
    
    
    refs.setSnapshot = setSnapshot;
    
    const maybeNewVersion = getVersion(source._source);
    if (!is(version, maybeNewVersion)) {
      const maybeNewSnapshot = getSnapshot(source._source);
      if (__DEV__) {
        if (typeof maybeNewSnapshot === 'function') {
          console.error(
            'Mutable source should not return a function as the snapshot value. ' +
              'Functions may close over mutable values and cause tearing.',
          );
        }
      }
      if (!is(snapshot, maybeNewSnapshot)) {
        setSnapshot(maybeNewSnapshot);
        const lane = requestUpdateLane(fiber);