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 [normalPri, setNormalPri] = useState(0);
      const [transitionPri, setTransitionPri] = useState(0);
      updateTransitionPri = () =>
        startTransition(() => setTransitionPri(n => n + 1));
      updateNormalPri = () => setNormalPri(n => n + 1);
      useLayoutEffect(() => {
        Scheduler.log('Commit');
      });
      return (
        <Suspense fallback={<Text text="Loading..." />}>
          <Text text={'Transition pri: ' + transitionPri} />
          {', '}
          <Text text={'Normal pri: ' + normalPri} />
        </Suspense>