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 [transitionPri, setTransitionPri] = useState(false);
      const [normalPri, setNormalPri] = useState(0);
      updateTransitionPri = () => startTransition(() => setTransitionPri(true));
      updateNormalPri = () => setNormalPri(n => n + 1);
      useLayoutEffect(() => {
        Scheduler.log('Commit');
      });
      return (
        <Suspense fallback={<Text text="Loading..." />}>
          {transitionPri ? <AsyncText text="Async" /> : <Text text="(empty)" />}
          {', '}
          <Text text={'Normal pri: ' + normalPri} />
        </Suspense>
      );
    }
    const root = ReactNoop.createRoot();
    await act(() => {
      root.render(<App />);
    });
    
    assertLog(['(empty)', 'Normal pri: 0', 'Commit']);
    expect(root).toMatchRenderedOutput('(empty), Normal pri: 0');