My wife and I have been experiencing problems with geofences with our iPhone 13 Pro and iPhone 13 Pro Max, both of them running iOS 15 out of the box.
I have issues with ally my geofencing apps (Locative, Geofency, IFTTT app, Tado), which do not execute actions upon entering/exiting geofences unless I open these apps almost every day. Tado failing to update is an issue, as it controls my heating.
Issues persisted with iOS 15.1.1, so I have just installed iOS 15.2 beta 3... let's see if problems have been solved.