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`.
useInsertionEffect: mountInsertionEffect,
  useMemo: mountMemo,
  useReducer: mountReducer,
  useRef: mountRef,
  useState: mountState,
  useDebugValue: mountDebugValue,
  useDeferredValue: mountDeferredValue,
  useTransition: mountTransition,
  useMutableSource: mountMutableSource,
  useSyncExternalStore: mountSyncExternalStore,
  useId: mountId,
};
if (enableCache) {
  (HooksDispatcherOnMount: Dispatcher).useCacheRefresh = mountRefresh;
}
if (enableUseHook) {
  (HooksDispatcherOnMount: Dispatcher).use = use;
}
if (enableUseMemoCacheHook) {
  (HooksDispatcherOnMount: Dispatcher).useMemoCache = useMemoCache;
}
if (enableUseEffectEventHook) {
  (HooksDispatcherOnMount: Dispatcher).useEffectEvent = mountEvent;
}
const HooksDispatcherOnUpdate: Dispatcher = {
  readContext,
  useCallback: updateCallback,
  useContext: readContext,
  useEffect: updateEffect,
  useImperativeHandle: updateImperativeHandle,