^^ We are talking about getting “unlimited frame rates with tearing”, not just about “tearing”. DX12 only allows getting that in windowed fullscreen mode. Again, ask Microsoft. Or NVIDIA.
Added: BTW, Wigz, I went through your reddit link and I get what it is about the timer. The achievement uses (or used, don’t know how it is currently) custom code to count how long you spent and that code could have been stalled by a simple CPU-eating loop. No wonder it could have also been stalled by something like switching to a different application (and not getting render calls).