Apple has just confirmed this to me that this is a known issue in iOS 15 and it should be fixed in iOS 15.1. We are experiencing the same issue, so fingers crossed.

Same here. Testing iOS 15.2 beta 3 and still the same issue. Given that this problem has apparently existed for 5 months it is quite troubling. So far what we have found is that if our app has been launched into the foreground recently (like in the last 24 hours or so), everything works as expected with region monitoring. However, if the user has not interacted with our app within the last day or so, all region events cease to function. On iOS 13 and 14 the exact same version of our app performed as expected. We could not launch the app for a month and region events would still function.