That’s a square not a triangle… But even if it would be a triangle doesn’t matter. I am switching the elements on different floors and it’s still work.
Also if the wall has 100% stability it should be possible to place a square or triangle at this point.
@TeleTesselator did you check with the repairhammer the stability? Sometimes I have the problem that I become 0 stability for no reason. And then nothing could be build anymore. Few month ago when I take this 0 stability items back to my inventory I get it twice back.