Gears of Wars 4, Forza and Horizon, Quantum Break, and most of te "big" games that have been released since Battlefront (ReCore is presumably DX11.x tho.. ) now are those games based on engines specifically build for DX12? Nope. besides maybe ForzaTech. (UE4 doesn't seem so bad too given that the DX12 path was actually mainly developed by Microsoft themselves when developing Fables Legends along with the sync Compute support and other things )
Forza doesn't use DX12 : http://wccftech.com/turn-10-studios-excited-dx12-utilizing-forza-6-xbox/
Also, its not always clear if DX12 is only available on the PC version.