i do own the steam version of the game.
if i browse the game folder manualy and start the dx12 exe, the benchmark will show the game running in dx11, and the runlog.txt will also show "executable path: ..\ashes_dx11
i was able to start dx12 only by renaming the dx11.exe to something random, and then renaming dx12 to dx11. the game now shows dx12 in the benchmark screen, runlog.txt of course still shows the same path.