Hi Yanzco,
This is a known issue with Orthographic view caused with lighting. You can vote on and track the issue here on our public tracker: Unreal Engine Issues and Bug Tracker (UE-11446)
In order for this to be gone, either use statically lit environments with no stationary or movable lights, adjust the roughness of the material to have not have any reflections or not use lighting and make the materials use the unlit option.
-Tim