I have the same problem (see picture below).
Using "Microsoft Whiteboard" I drew lines with the Surface Studio 2 (red) and an iPad Pro (blue).
Since I draw a lot and I'm used to draw on a glassy surface, my freehand lines (1) are normally pretty straight.
In order to eliminate jitter I used a physical ruler on either the Surface Studio 2 and the iPad Pro (2).
The straight lines can only be produced when using the built in software ruler (3).