I am experiencing heightfield seams at the borders of world composition landscapes. The terrain heightfield was generated in worldmachine, and converted to raw files using Photoshop, and saved as .r16.
These were brought into the level using the world composition landscape importer, with a scale of 125 on each dimension. This looks like a straight up precision issue, which could be in the data, or the Editor itself.
I was able to reproduce this and have entered a bug report, UE-12189 to be entered by the development staff. As a workaround, you can use the smoothing tool with a strength of 0.0 and run that over the seam, which should seal it without smoothing out the landscape details.