Sorry for so late reply. Thanks for the advice, but I've already written in the Steam technical support thread and asked other players - there's still no solution to the problem in fullscreen mode with DX 11. The devs, apparently, are not going to fix it (or maybe it will be fixed in patch №7).
But I found another way to launch the game with DX 11 in the correct resolution - you just need to change "fullscreen" to "windowed" (without borders).
