The issue has been fixed with iOS 15.0.1

Thank you for the response. I will file a Feedback report as suggested. One complication is that the issue does not occur on my own devices. However I receive approximately 20–30 user reports per week with identical symptoms and my internal logging clearly confirms that locationManager(:didVisit:) stops being invoked entirely once the issue appears on a device. After that point no visit events are delivered anymore for that installation unless the user deletes the app and reinstalls it from the App Store. Because the problem only affects end-user devices I currently do not have device-side system logs such as sysdiagnose or logs generated with the Location Services profile. I will still submit a Feedback with all information I have collected. If additional diagnostic data becomes available from affected users I will add it to the Feedback. I will post the FB number here once it is created.

FB21022960