The reason those blue triangles appear is that CFS3 engine incorrectly assumes that those terrain tiles should be out of view and doesn't render them. It doesn't depend on GPU or your config, it is purely a bug in their algorithm which runs on CPU.
I don't have a direct solution for this, but I have something that may reduce the issue and improve the performance at the same time.
I've already mentioned this tweak on sim-outhouse some time ago but it had little effect on the "stock" CFS3, however I believe it may work better WOFF because WOFF terrain is more detailed.
You can try it if you feel adventurous smile

Here is a zip file with compositetexturebudgets.xml - the file goes into WOFF\OBDWW1 Over Flanders Fields\terrains folder, but don't forget to make a backup of the original file there.
https://1drv.ms/u/s!AvmtwF9vPtwVgYoiGbgXo8KXiEzxUQ

Additionally you should edit shaders30\Terrain.fx file and change 256 to 1024 in this line near the beginning of the file:
const static float fTextureSize = 256;
This is needed for terrain bump mapping to work properly with modified budgets.

I would have made a JSGME package but I don't remember how.

PS: And yes, I'm working on DX11 version of shaders smile