That's not a bug or something that needs to be fixed. Due how the ground is made at KDFW (the 1st scenery ever to feature shaders on ground polygons), it's not compatible with AI shadows *after* your viewpoint is higher than a certain height.
Because of this, since you wouldn't be able to see the airplane shadows anyway, the scenery automatically turn it on/off if your viewpoing is below/above a certain height threshold.