Can't give a proper answer as not using Libre at the present time, though it was working with a new iPhone SE. The iPhone XS is certainly on the list of compatible phones. It may be worth going to Settings to check Bluetooth is switched on. It wasn't by default when I took the phone out of the box. If the apps have been transferred from the iPhone 7, it may be worth checking you have version 2.5.3.3088 or later.