I was playing with a buddy and the terrain I flattened behind our house would either appear as a heap or a pit for him. Logging out and back in sort of reset it to where we're looking at the same thing, with the heap reappearing for me. While I could not see the heap and he could, he could still interact with it by standing on top and appearing to float in mid air to me. Another issue is that this means that the world save did not save my terrain changes as host. This is extremely frustrating and I'll either abandon the save or the base as it's messing up the smelter area.