I have the same issue myself. I just happened on this post. I have an underground vault. I used the Clear Terrain tool to create an opening in the landscape. Once you cross that threshold where you go under the landscape, you cannot shoot.
I have the area covered in an UndergroundVolume and it did not help. I also added an WaterBodyExclusion and it did not help. Then I tried adding a mutator zone that allows shooting and still did not work.
I tried all 3 volumes individually by themselves, and all possible combinations of them together, nothing allows me to shoot under the landscape.