iOS11: airplane mode and Bluetooth
I had the same issue but I finally found out how to fix it. Bluetooth actually stays on the whole time even if in the control center the bluetooth button isn’t even blue. 
Go open “settings”, and you’ll see that bluetooth is switched on (and maybe you thought it was off like in my case). If you turn it off in the settings, the bluetooth icon from the control center will get crossed, and from there it doesn’t get switched on automatically every time you disable airplane mode.

Good Luck :)