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`.
seededCache.data.set(seedKey, seedValue);
          } else {
            if (__DEV__) {
              console.error(
                'The seed argument is not enabled outside experimental channels.',
              );
            }
          }
        }
        const payload = {
          cache: seededCache,
        };
        refreshUpdate.payload = payload;
        return;
      }
    }
    provider = provider.return;
  }
  
}