If the map in your OS Maps app disappears when you try to over-expand or zoom in on your iPhone, it may be due to a bug or a settings issue. Here are the steps you can take to troubleshoot and resolve this problem:

1. Check for App Updates
Ensure that you are using the latest version of the OS Maps app. Go to the App Store, type "OS Maps" in the search bar, and see if there are any updates available. If there are, update the app.

2. Enable Location Services
Sometimes, missing map data can be linked to location services settings. Follow these steps to enable location services for the OS Maps app:
- Go to your iPhone's Settings.
- Scroll down and tap on Privacy.
- Select Location Services.
- Make sure that Location Services are turned on and find OS Maps in the list to ensure that it has permission to access your location. - learn more

3. Reinstall the OS Maps App
If the map still disappears after updating and checking location settings, try uninstalling and reinstalling the app:
- Press and hold the OS Maps app icon on your home screen until the icons shake.
- Tap the X to delete the app.
- Reinstall the app from the App Store.

4. Adjust Map Settings
After reinstalling, explore any in-app settings that may affect how maps are displayed. Check if there are options related to map visibility, layers, or zoom levels.

5. Clear App Cache (If Applicable)
If the app has a feature to clear cached data, consider using it. This can sometimes resolve glitches that lead to disappearing maps.

6. Check Internet Connection
Ensure that you have a stable internet connection, as network issues can sometimes lead to maps not loading correctly.

7. Contact Support
If the issue persists, consider reaching out to OS Maps support for assistance. There may be a specific bug affecting your version of the app that they are aware of and can help you fix. - learn more

By following these steps, you should be able to resolve the issue with the disappearing map in the OS Maps app on your iPhone.