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`.
if (typeof create !== 'function') {
      console.error(
        'Expected useImperativeHandle() second argument to be a function ' +
          'that creates a handle. Instead received: %s.',
        create !== null ? typeof create : 'null',
      );
    }
  }
  
  const effectDeps =
    deps !== null && deps !== undefined ? deps.concat([ref]) : null;
  updateEffectImpl(
    UpdateEffect,
    HookLayout,
    imperativeHandleEffect.bind(null, create, ref),