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`.
const record = data.get(text);
        if (record === undefined) {
          const newRecord = {
            status: 'rejected',
            value: error,
          };
          data.set(text, newRecord);
        } else if (record.status === 'pending') {
          const thenable = record.value;
          record.status = 'rejected';
          record.value = error;
          thenable.pings.forEach(t => t());
        }
      },
    };
    caches.push(cache);
    return cache;
  }
  function readText(text) {