placing walls on triangle bases.

Its easier just to show you, you can place walls next to each other around a circle of traingles on ground floor 

but when you try it higher up the walls cannot be placed next to each other and say "too close to another building". 

Iv'e tried reversing the walls, using frames instead, and there is nothing other than the walls in any kind of proximity. Wall placement like this has always worked so this is a new bug. And it isn't my location because I know of another player who had the same prob. And sometimes if you case a wall from a different angle or look down from above it works, but other times nothing works.

Thanks