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`.
isSafeToReadFromSource = isSubsetOfLanes(
      renderLanes,
      root.mutableReadLanes,
    );
    if (isSafeToReadFromSource) {
      
      
      
      setWorkInProgressVersion(source, version);
    }
  }
  if (isSafeToReadFromSource) {
    const snapshot = getSnapshot(source._source);
    if (__DEV__) {
      if (typeof snapshot === 'function') {
        console.error(
          'Mutable source should not return a function as the snapshot value. ' +