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 [show, setShow] = useState(false);
      const [isPending, _start] = useTransition();
      start = () => _start(() => setShow(true));
      return (
        <Suspense fallback={<Text text="Loading..." />}>
          {isPending ? <Text text="Pending..." /> : null}
          {show ? <AsyncText text="Async" /> : <Text text="(empty)" />}
        </Suspense>
      );
    }
    const root = ReactNoop.createRoot();
    await act(() => {
      root.render(<App />);
    });
    assertLog(['(empty)']);
    expect(root).toMatchRenderedOutput('(empty)');
    await act(async () => {
      start();
      await waitForAll([
        'Pending...',
        '(empty)',
        'Suspend! [Async]',
        'Loading...',
      ]);
      expect(root).toMatchRenderedOutput('Pending...(empty)');
      await resolveText('Async');
    });
    assertLog(['Async']);