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`.
__DEV__ &&
    (currentlyRenderingFiber.mode & StrictEffectsMode) !== NoMode
  ) {
    mountEffectImpl(
      MountPassiveDevEffect | PassiveEffect | PassiveStaticEffect,
      HookPassive,
      create,
      deps,
    );
  } else {
    mountEffectImpl(
      PassiveEffect | PassiveStaticEffect,
      HookPassive,
      create,
      deps,
    );
  }
}
function updateEffect(
  create: () => (() => void) | void,
  deps: Array<mixed> | void | null,
): void {