This happens because you are in Record: Box mode, which requires only two points to form a box. If you don’t form a box when in Box mode then you will draw what looks like a line but is actually a box (with one side of the box on top of the other side). Box and Point To Point modes are visible at the bottom of the PlanSwift window only when Digitizer Record is on. The Record Mode button toggles between Box and Point-to-Point.
