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`.
setCurrentUpdatePriority(previousPriority);
    ReactCurrentBatchConfig.transition = prevTransition;
    if (__DEV__) {
      if (prevTransition === null && currentTransition._updatedFibers) {
        const updatedFibersCount = currentTransition._updatedFibers.size;
        currentTransition._updatedFibers.clear();
        if (updatedFibersCount > 10) {
          console.warn(
            'Detected a large number of updates inside startTransition. ' +
              'If this is due to a subscription please re-write it to use React provided hooks. ' +
              'Otherwise concurrent mode guarantees are off the table.',
          );
        }
      }
    }
  }
}
function mountTransition(): [
  boolean,
  (callback: () => void, options?: StartTransitionOptions) => void,
] {
  const [isPending, setPending] = mountState(false);
  
  const start = startTransition.bind(null, setPending);
  const hook = mountWorkInProgressHook();
  hook.memoizedState = start;
  return [isPending, start];
}
function updateTransition(): [
  boolean,