I also have this problem. As you described it, that means I want to build a 45 degree piece, the blueprint is okay, then confirm and the blueprint jumps to straight.
Or the blueprint is okay, after confirming it, the Blueprint jumps 2-3 degrees (floor, walls and stairs).
But this only after a while. I can build 3 - 4 floors normally, then it starts.
Please fix this Obsidian.