"No service" message when on WiFi. By design or bug?

On our sample device the status bar shows "No Service" (see attached picture) whenever the device is connected to a wireless network, even if there is also a valid phone connection and the device can still make and receive calls and texts.  As soon as the wifi connection is turned off, the normal network provider details (in this case EE) appear again.
Is this the expected behaviour or should I raise a bug report somewhere for this?