iPad Pro AirPlay mirroring to Apple TV not full screen

Trying to use iPad Pro with AirPlay mirroring to AppleTV (both Gen 1 & 4) but image only fills part of the screen as if it's 4:3 instead of 16:9.

Works perfectly on both the Gen 1 & 4 TV using my iPhone 6.

Is this a settings issue or a design issue?

Thanks.