You know that darkened rim around the screen you're working on that shows the combos on the edge of the screens around it? The left side of that seems to get confused with dungeon door sets.
For an example, open up proposed1st.qst and visit Map 3, Screen 31. See how there's a weird bombed door on the left side of the screen? It should be a shutter, and it should be facing the opposite direction. Lots of other screens around the dungeons also have this problem. Try Map 3, Screen 00 for example. Even though it's the top-left-most screen, there's still a bombed door to its left!
I don't know what specifically is causing this, but at least it's very easy to observe. I can't seem to get it to happen with the three other sides of the preview rim (up, right, and down), so it seems fairly isolated, too.