That's because you don't need an adapter to use DisplayPort with TB3 (on the MBP). Just purchase the correct [1] cable; the MBP implements DP as an alternate mode.
That cable isn't available on this side of the Atlantic, local Apple store only had a faulty DP adapter. For some reason not even the german Amazon had the proper cable in stock or listed.
I want ONE CABLE for everything. That was the promise, I thought. I want 2x4k@60hz + power + 3 USB3 with one connector going to the tbMBP. Apparently this is not possible.