Hello! I understand you're having trouble with completed games disappearing from the calendar on Sudoku.com after you close and return to the app. To help you best, could you please tell me if you are playing Sudoku.com through a web browser or via an app? Also, have you noticed if the problem happens consistently with every game or intermittently?

Meanwhile, here are some initial steps we can try since you’re on a Windows machine using WebKit browser:

1. Check your internet connection: Because your game progress and calendar data sync online, a stable connection is essential for saving your completed games.

2. Ensure you are logged into your Sudoku.com account: Progress is usually saved under your account. If you play without logging in, your progress may not be saved.

3. Clear your browser cache and cookies: Sometimes corrupted cache can affect saving features. You can clear this from your browser settings under privacy or history options.

4. Avoid closing the session abruptly: Make sure to properly close or exit your game or the puzzle after finishing before closing the browser tab.

5. Update your browser: Sometimes an older browser version can cause compatibility issues.

If you have tried these steps or if the problem persists, I can guide you through more advanced troubleshooting or check if there is a known service issue.

Are you comfortable trying these steps, and can you share any additional details such as how you access Sudoku.com?