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`.
'Transition pri: 1',
      ]);
      
      
      updateNormalPri();
    });
    if (gate(flags => flags.enableUnifiedSyncLane)) {
      assertLog([
        'Normal pri: 0',
        'Commit',
        
        'Transition pri: 1',
        'Normal pri: 1',
        'Commit',
      ]);
    } else {
      assertLog([
        
        'Normal pri: 0',
        'Commit',
        
        'Transition pri: 1',
        'Normal pri: 1',
        'Commit',
      ]);
    }
    expect(root).toMatchRenderedOutput('Transition pri: 1, Normal pri: 1');
  });
});