Floor Interference With Objects
Here's a screenie of what I'm referring to.
If I lay a floor in the corner, the curved arch will not fasten to the foundation. The arch is custom. The straight arched walls are vanilla, and they go down just fine.
However, some vanilla assets have this same problem. Try installing the large wood-and-stone portcullis over floors. Won't happen. Remove the floor from the blocks and the problem goes away.