While maybe not being consistent behaviour between Apple devices, I'd have thought this was expected behaviour. When an app is put in the background, it'll eventually be stopped from using up battery and comms resources, so its Wifi access will be cut off. Personally I'd have thought that putting the app in the background would cut off the Wifi access from it fairly quickly, so 5 minutes is quite a long time to have a background app still communicating over Wifi!