I was able to get rid of the splotches by increasing the Static Lighting LOD level.
I started by loading up World Machine and exporting one of their example maps heightmaps. Linked below
Next I imported that and used the default settings. I then scaled the map in the Z axis to be ~30.
Next I built everything on Production Lighting Quality for the first image.
Then I set the Static LOD Lighting to 5 and built (Tried different levels until this one removed the splotches)
Here is the before shot with Static Lighting LOD at Default 0:
Here is the after with the Static Lighting LOD set to 5:
I hope this helps!
Tim