Yes, there is definitely an issue with geofencing on iOS 15.
Our app (mileage tracker) relies on geofencing / region monitoring and has worked flawlessly until iOS 14.8. After updating to iOS 15 (as well as 15.0.1 and 15.0.2), the app stopped receiving regular region exit and/or region entry events, and is no longer launched to the background when the user moves, even a significant distance. This happens only on some devices, while on others there is no issue (!?)
When the issue occurred, restarting the device helped some users, but did not help others.
Even for a given user, on a given device, region monitoring would work perfectly well on one day, and fail completely on another day, causing all trips on that day to be missed by our app.
We were able to reproduce the issue on some test devices and reported it to Apple on 8 October 2021 (Feedback ID FB9687008). We have not yet heard back from Apple about this ticket.
On one of our test devices, restarting the iPhone helped "resolve" the issue and we have not been able to reproduce the issue on that device again. On other devices, users have reported that even after multiple restarts the behaviour is unpredictable. Geofencing works on one day, then not working on the next day...
Really frustrating, and no word from Apple on this so far!