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 [shouldHideInParent, _setShouldHideInParent] = useState(false);
        setShouldHideInParent = _setShouldHideInParent;
        Scheduler.log('shouldHideInParent: ' + shouldHideInParent);
        if (shouldHideInParent) {
          return <Text text="(empty)" />;
        }
        return children;
      }
      let setShouldHideInParent;
      function App() {
        const [shouldSuspend, _setShouldSuspend] = useState(false);
        setShouldSuspend = _setShouldSuspend;
        return (
          <>
            <Text text="A" />
            <Parent>
              <Suspense fallback={<Text text="Loading..." />}>
                {shouldSuspend ? <AsyncText text="Async" /> : null}
              </Suspense>
            </Parent>
            <Text text="B" />
            <Text text="C" />
          </>
        );
      }