This is a minor one. The perspective in the OpenGL-Renderer is just a little bit off. When you look straight up or down and rotate your view by 90 degree, you can see Texture warping (Look at the example screenshots and video in the attachments).
TFE uses a standard projection matrix, so looking up and down should look like other 3D games. Though one thing to note is that the maximum angle is slightly less than 90 degrees because the camera does need a slight look direction, so the ground will never look perfectly flat even at maximum pitch - which might be what you are seeing.