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`.
init?: I => S,
    ): [S, Dispatch<A>] {
      currentHookNameInDev = 'useReducer';
      warnInvalidHookAccess();
      updateHookTypesDev();
      const prevDispatcher = ReactCurrentDispatcher.current;
      ReactCurrentDispatcher.current =
        InvalidNestedHooksDispatcherOnUpdateInDEV;
      try {
        return updateReducer(reducer, initialArg, init);
      } finally {
        ReactCurrentDispatcher.current = prevDispatcher;
      }
    },
    useRef<T>(initialValue: T): {current: T} {
      currentHookNameInDev = 'useRef';
      warnInvalidHookAccess();
      updateHookTypesDev();
      return updateRef(initialValue);
    },
    useState<S>(
      initialState: (() => S) | S,
    ): [S, Dispatch<BasicStateAction<S>>] {
      currentHookNameInDev = 'useState';
      warnInvalidHookAccess();
      updateHookTypesDev();
      const prevDispatcher = ReactCurrentDispatcher.current;
      ReactCurrentDispatcher.current =
        InvalidNestedHooksDispatcherOnUpdateInDEV;
      try {