I'm not sure what the glitch is or why it occurs. Imagine a normal repeating terrain. Now take all of the repeats and make them invisible, but let the player camera still collide with them. It looks a bit like that. Not exactly of course, I don't think the camera follows the counters of the terrain repeats.
You'll only notice it in third person, not first person.
Also, I really don't think multiple terrains will work, you will have to use a large terrain and the set empty tool.