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`.
expect(root).toMatchRenderedOutput(
        <>
          B label (loading...)<div>A content</div>
        </>,
      );
      
      await act(() => {
        update('C');
      });
      assertLog([
        
        'C label (loading...)',
        'A content',
        
        'C label',
        'Suspend! [C content]',
        'Loading...',
      ]);
      expect(root).toMatchRenderedOutput(
        <>
          C label (loading...)<div>A content</div>
        </>,
      );
      
      
      await act(() => {
        resolveText('B content');
      });
      assertLog([