I just switched my Surface Book Dock to dual mDP - DP cables and I cannot get my second monitor to work properly. I can see both in the Device Manager but not in the Display Properties. Both monitors work on mDP - HDMI, Help!!!!
I did something similar once, tried using DP cable instead of the Dvi cable I had been using. Windows got totally confused with the monitors and despite deleting them just reused the old settings and would not display properly on them. I ended up doing a complete Refresh reinstall to get them working again.
Later someone (I think @surfdock) posted a tool to clear the display settings which I think would have worked but I don't have it and cant find it.