I am also facing this same issue where my Live Activity doesn't appear in iOS 17.6.1 and 17.7.1 when I build the app with Xcode 16.0 (16A242d) if @Environment(.isActivityFullscreen) is used. Using Xcode 16.1 (16B40) doesn't fix the problem either. Did not try 16.2 beta

This is fixed for my app with iOS 16.1 (20B5045d) running on a simulated iPhone 14 Pro from Xcode 14.1 beta (14B5024h)