Your surface can’t extend beyond the visible area, the part that’s not visible will receive shadows, so you have a floor with a wall on top of it, but the floor goes underneath the wall. When you build the lighting pixels that are lit will extend underneath the wall and might be visible on the other side, or you might get a shadow underneath the wall which can cause dark pixels to appear on the floor since the mesh can’t line up perfectly with the lightmap.
It’s not worth the trouble to try and line up meshes with lightmap pixels, since you can’t do that with meshes that aren’t rectangular.