I’ve got this on PC too, made a thread like this aswell aha
You can use a floor (flat ceiling or wedge) to build from instead of the triangle, make it so the floor’s edge is lined up along the top of the triangle so anything you build onto it is actually connected to the floor rather than the triangle peice. It can look ugly though.
I also managed to build a wall on top of the broken inverted triangle peice by building a roof and two walls either side, but everytime I load in the wall instantly brakes like it has 0 structural integrity. The only fix right now that I can see is the using a floor/ceiling peice)))