minosimu_nz The walls dropping under stairs is not a bug. The game has always done that when you have the walls set to both cutaway and wall down views in both live and build mode in a vanilla (no mods/no CC) game. This is how it is designed to work. I did notice that when I tested in my main save where I do use both mods and CC, those walls did not drop in cutaway view in build mode only. I don't know why that happened, only that it did and can only assume that I have something in my mods folder that caused it to act that way. Since you are not using any mods or CC, what you show in your pic is normal though.
Hope this helps.