identifierForVendor changes on reinstall

It's a known bug. It seems like Apple made an update to AppStore that causes this new behavior for identifierForVendor around the 28:th May. If you search in the App Developer forum, there are other developers reporting the same problem.

The signature gc from Apple have replied on the issue with the following answer: "Please file bug reports on this at https://developer.apple.com/bug-reporting>. We're aware of this issue and are investigating. There's no known workaround at this time."

identifierForVendor is expected to change when all vendor's apps are removed from the device. Also, it is bound to change if you're building and installing from Xcode directly.

The value in this property remains the same while the app (or another app from the same vendor) is installed on the iOS device. The value changes when the user deletes all of that vendor’s apps from the device and subsequently reinstalls one or more of them. The value can also change when installing test builds using Xcode or when installing an app on a device using ad-hoc distribution. Therefore, if your app stores the value of this property anywhere, you should gracefully handle situations where the identifier changes.

Source: UIKit/UIDevice/identifierForVendor