WiFi Disconnects when Lock Screen active

Whilst at Home, I've been wondering why with the iOS 11 release, I keep seeing messages saying that such and such an application doesn't have access to Mobile data, particularly when unlocking my device. Being at home, I'd have expected my iPhone to be constantly connected to my Home WiFi.

So, as a test, a ran a simple "ping" command from my PC, to the IP Address of my iPhone whilst it's attached to my Home Wireless router. Shortly after the iPhone screen goes dark, the Ping results start showing "Request timed out." followed by "Destination host unreachable". This continues until I press the Home button again.

So clearly, whilst the iPhone is in Lock-screen mode, the WiFi is disconnected. This didn't happen with Prior versions of iOS, so why now ?

As a consequence of this, iPhone system apps will now use the Carrier's 3/4G network for data (as you can't disable them) and any other application that you may have specifically permissioned for Mobile data access will be able to chew up your Carrier's data allowance. This explains why I've been seeing my data Mobile data usage creep up even though I haven't been away for WiFi.

Why is this now happening, it's surely a problem - particularly if you stream music overnight whilst you sleep.