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 (__DEV__) {
      console.error(
        'Expected a constant size argument for each invocation of useMemoCache. ' +
          'The previous cache was allocated with size %s but size %s was requested.',
        data.length,
        size,
      );
    }
  }
  memoCache.index++;
  return data;
}
function basicStateReducer<S>(state: S, action: BasicStateAction<S>): S {
  
  return typeof action === 'function' ? action(state) : action;
}
function mountReducer<S, I, A>(
  reducer: (S, A) => S,
  initialArg: I,
  init?: I => S,
): [S, Dispatch<A>] {
  const hook = mountWorkInProgressHook();
  let initialState;
  if (init !== undefined) {
    initialState = init(initialArg);
  } else {
    initialState = ((initialArg: any): S);
  }
  hook.memoizedState = hook.baseState = initialState;