A problem with Landscape Visibility Mask.
Landscape wa created with World Machine with it own customized level streaming.
The Material works fine, the only thing that it is impossible to make holes in terrain.
I am currently having the same issue with a tiled build from world machine, i have the visibility mask in the material and attached but get a note stating i need to connect it in the editor when trying to adjust visibility. I am on 4.19 not sure how to fix or whats causing the problem. Stated above it works fine on a landscape created in engine.
I was able to fix this issue with the world composition import from world machine by applying the visibility material to the actual landscape actor in the World Outliner. it can not be applied to the Landscape Streaming Proxy and work. Hope that helps someone else.