Apparently, every iOS device that has been updated to the 11.4 version has been affected by it. According to GSMarena, while switching off the Wi-Fi worked for some users, connecting to a 2.4GHz Wi-Fi network (instead of 5GHz) helped several others save battery. There is a third group of users who have switched back to iOS 11.3.1 to keep their phones running for longer hours before they plug-in the charger.