Incorrect stability calculations

Game mode: [Online | Singleplayer]
Problem: [Bug]

I built big roof over my map room and I have problem. I can’t build tiled sloped roof on flat roof because of stability.
Game says me this flat roof has 20 stability points but I build pillar to support this. Interstingly the roof next to this one have 80 stability points.

Steps on how to reproduce issue:

  1. Just build roof over map room