Thiis is actually due to the top-left bridge uprights being fmasks with the illusion of height. This means the graphics break the tile boundaries so such an fmask in tile x,y will 'overflow' to tile x,y-1 (and further depending on how much height it has). Due to the order in which the map is drawn, if tile x,y-1 has an fmask of its own this is drawn over the top of the overflow.
You can see this at either end of the bridge too where green grass dots are drawn on top of the bridge uprights. But of course that is far less noticeable.
It occurs to me that we might be able to exploit the nature of isometric graphics to eliminate this problem simply by drawing the tiles in reverse order. Not sure though.
EDIT: Or are those floors with height? Same difference whatever.