While trying to use the MikroBUS socket for the DA14531 on the "Development Kit-Pro" board I did not get the same result as with the "Development Kit-USB". After some searching, I have found that the P21 on J17 is left unconnected, confirmed by the published schematics. I think this is a hardware bug because I don't see any reason to do so.
I solvedt this problem by soldering a wire between P21 on J17 and P21 on J2. See the photo in the DevKitPro-P21.pdf file.