There was an issue recently where apps were being given an incorrect identifierForVendor when they were installed or updated. This was a bug that was introduced near the beginning of May. Around July 21, the App Store completed the process of restoring the identifierForVendor of apps that experienced this bug to the value it had before the bug was introduced.
Apps that were installed or upgraded while this bug was present will see one more identifierForVendor change the next time the user updates or reinstalls the app after about July 21 but it will be a value the app had used before. (The data used to compute identifierForVendor is delivered along with app installations or updates.)
IMPORTANT: This means that you may get random reports about this issue because it only appears when the user updates your app.
If you haven't updated your app since about July 21, you may want to consider shipping an update of your app so the user can perform an update without removing your app and thus losing data. This update wouldn’t have to have any different functionality.
There should be no further identifierForVendor changes once a user updates your app after about July 21 unless the user removes all of your apps and then reinstalls one of them, or if they restore their device from a backup created on a different device. These are the documented cases where identifierForVendor is expected to change.
If you see any behavior other than what I’ve described, please file a bug report about it at <https://developer.apple.com/bug-reporting> so we can investigate.