https://motherboard.vice.com/en_us/article/bjvjd3/apple-doesnt-trust-you

On Wednesday we reported that turning off Bluetooth from iOS 11's Control Center—the quick access menu where users can toggle the device's most used features—doesn't actually turn off Bluetooth.

Instead, pressing the button will immediately disconnect the iPhone from Bluetooth accessories, but Bluetooth will continue to be available. That is because Apple wants the iPhone to be able to continue using AirDrop, AirPlay, Apple Pencil, Apple Watch, Location Services, and other features that Apple has touted as defining innovations over the last couple of iterations of its mobile devices.