I think for Kaalus to fix this, he would need to redesign the rendering engine, which is a huge task. This really dissapointed me a lot as well. The first thing i designed, was roof tiles, which took quite a bit of time. To my utter "delight" I now could see the insides of my world. I think it has to do with the fact that these bitmap type designs are converted to vector graphics and the data becomes too much for the engine to handle. It could be that the integers used just run out of space, since integers are very limited. But even when using floating points, there are limits. Even the compilers might have limits to where these floating points can go, 32 bit? 64 bit? 128bit?
For now I hope Kaalus is focussing on proper multiplayer, then I can wait for this bug or rather restriction, to be sorted out. Technically, adding more and more furniture should only make the game slower, not mess up the graphics. I do have some mercy for Kaalus, since he is only one guy. I wish he would get in some help, even one guy would be great. Maybe one of his children will soon be able to program well and help him. His son seems to be quite talented.