First of All use the Learn Tab → Landscape Mountains make sure you’r using the both in the version 4.12.x(5)
Second open the map and make a clone and save with the name you want, now in that map remove the landscape, and set all lights to total dynamic.(Directional and Skylight)
Now in the directional settings set the max of dynamic cascade shadows and max activate and set too the max of far cascade shadows.
Don’t change nothing more and save.
Now create or add new landscale with this heightmap https://dl.dropboxusercontent.com/u/28070491/UE/Forums/Report/inter%20Height%20Map%20(Merged).png don’t apply texture just the heightmap, and create the landscape make sure you center it more or less to the world origin (0,0,0). Make sure the landscape is about 8x8km in sides scale in the settings panel to fit more or less.
Until here this setup apply to my 2 reports, but now is different.
Now go down in the map and at the -500m you will see how the bug spawn.