What you see in the northern highlands can be best described as the “precursor” sandstorm effects, but not to the point where the display is completely washed out with the “high wind” effect. However, in the southern highlands you can actually get the full thing, just absent the DoT (essentially the same as what happens with the jungle).
In both practical gameplay and in terms of consistency with the lore, none of that should be happening. If the sandstorm was, in fact, affecting the highlands then the highlands would be … desert. Keep in mind (again, more lore) that Warmaker Klael lost control of the elemental. What we experience now as the desert used to be a lush, verdant countryside with forests, etc. The priest-kings hoisted themselves on their own petard, so to speak, and destroyed their own lands in an attempt to repel the invading humans and counter a new weapon they had created.
So, bottom line, the only place that the sandstorm should “be” would be in the areas that are already largely desert. Because the sandstorm is how they became a desert in the first place.