FCM is swizzling both the methods of UNUserNotificationCenterDelegate i.e. willPresentNotification and didReceiveNotificationResponse.
FIRMessagingRemoteNotificationsProxy.m

But while un-swizzling, only willPresentNotification method is un-swizzled and not didReceiveNotificationResponse.
FIRMessagingRemoteNotificationsProxy.m

Due to this, the method didReceiveNotificationResponse of notificationDelegate class always points to FCM even after notificationDelegate is changed to a different instance. This creates a lot of issues when there are multiple SDKs swizzling these methods.