I suspect you’re running into an issue in the Path tool. There is a bug in the Path tool in LayOut 2017 that makes it extremely easy to accidentally create a spline when trying to create a straight line. By “extremely easy” I mean “creates a spline even if you don’t move the mouse a single pixel between pressing and releasing the mouse button”. These splines are functionally straight lines unless you then make some edits (such as using the Split tool).
Stay tuned for a fix for this bug.