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`.
enqueueConcurrentHookUpdateAndEagerlyBailout(fiber, queue, update);
            return;
          }
        } catch (error) {
          
        } finally {
          if (__DEV__) {
            ReactCurrentDispatcher.current = prevDispatcher;
          }
        }
      }
    }
    const root = enqueueConcurrentHookUpdate(fiber, queue, update, lane);
    if (root !== null) {
      const eventTime = requestEventTime();
      scheduleUpdateOnFiber(root, fiber, lane, eventTime);
      entangleTransitionUpdate(root, queue, lane);
    }
  }
  markUpdateInDevTools(fiber, lane, action);
}
function isRenderPhaseUpdate(fiber: Fiber): boolean {
  const alternate = fiber.alternate;
  return (
    fiber === currentlyRenderingFiber ||
    (alternate !== null && alternate === currentlyRenderingFiber)
  );
}
function enqueueRenderPhaseUpdate<S, A>(
  queue: UpdateQueue<S, A>,