I was having difficulties when trying to subtract one shape from another:
(the selected triangle should appear as negative space) which seemed like a bug until I tried to flip the triangle horizontally:
Voila! Flipping produced reversing the path orientation and subtraction now produces a negative space. But what to do when the negative space is neither horizontally nor vertically symmetrical? How can be path orientation be reversed?
For those who are not aware of it, this method is used in digital typography to define negative spaces and requires no further actions. i.e in Glyphs.app:
I believe this is what you're looking for:
You seem to already have a handle of it, but here's a nice article explaining how Sketch deals with things.