Many iPhone and iPad users are annoyed at Apple’s decision to change the way Control Center’s Wi-Fi and Bluetooth controls work, as they no longer work. Fortunately, you can still switch connectivity off quite easily.
What is the problem?
Apple in iOS 11 decided that when you tap the Wi-Fi or Bluetooth buttons in Control Center, the system now will disconnect you from any devices or networks you are currently on but no longer truly switches Wi-Fi or Bluetooth off.
This means that even though you thought you switched them off, they remain active for things like AirDrop, AirPlay, Continuity, Hotspot, Location services and devices such as the Apple Watch and Pencil.
Source: Computerworld.com | Security