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`.
useImperativeHandle: updateImperativeHandle,
  useInsertionEffect: updateInsertionEffect,
  useLayoutEffect: updateLayoutEffect,
  useMemo: updateMemo,
  useReducer: rerenderReducer,
  useRef: updateRef,
  useState: rerenderState,
  useDebugValue: updateDebugValue,
  useDeferredValue: rerenderDeferredValue,
  useTransition: rerenderTransition,
  useMutableSource: updateMutableSource,
  useSyncExternalStore: updateSyncExternalStore,
  useId: updateId,
};
if (enableCache) {
  (HooksDispatcherOnRerender: Dispatcher).useCacheRefresh = updateRefresh;
}
if (enableUseHook) {
  (HooksDispatcherOnRerender: Dispatcher).use = use;
}
if (enableUseMemoCacheHook) {
  (HooksDispatcherOnRerender: Dispatcher).useMemoCache = useMemoCache;
}
if (enableUseEffectEventHook) {
  (HooksDispatcherOnRerender: Dispatcher).useEffectEvent = updateEvent;
}
let HooksDispatcherOnMountInDEV: Dispatcher | null = null;
let HooksDispatcherOnMountWithHookTypesInDEV: Dispatcher | null = null;
let HooksDispatcherOnUpdateInDEV: Dispatcher | null = null;
let HooksDispatcherOnRerenderInDEV: Dispatcher | null = null;