Spandrel in rounded room area placing incorrectly

I'm trying to add a spandrel to the rounded room area and it keeps going into places I don't want it to. I am unsure why the options outside the wall itself are even available as I have not built anything that shape in that area. I know it has to be something I've done wrong but I just don't know what. I think I attached the picture. No cc or mods in my game.

[CM edit - descriptive title]