Whenever I use a texture that has some transparent parts like a grate. The parts are not see through. This happens with models like barbwire.
The models are good in the game but when I am making them they look like this
Another way to move the new cut up wall back, is to turn it into a prefab. Then you can go back to the original, delete the old wall and insert the prefab one. As long as you didn't move the cut up wall in the new map, it will end up where it started.
I already did the prefab thing and now I have another question.
I have lod terrain and when I tested it in the game I could walk throught certain patches of terrain. (Clip brushes maybe)
There is a difference between LOD Terrain and Patch Mesh. But basically, the texture you use on them is what makes them solid. So, try another texture.
Some of the time it is just a bug . I had a snow map that had a spot like that so I just use a snow caulk bush where the hole was . With patch meshes you use the [ and ] keys to reduce or make more complex the patch , reduce too much and you start falling though it but you can make it more complex to fix the problem .