Just a thought: if it's a bug, how should the fog of war act when you want to build there? Not allow you until you have LoS on the whole building area? Generally a bug is something (code) that has unexpected behaviour/output. I think the output of not allowing to build, even with fog of war present, on a place where a unit or a building exists is part of how the gameplay was thought through. Yes, this has other consequences, but I don't think they can be called a bug in the game.