It’s not a bug, it’s absolutely intentional.
The correct path for a round cutting tool, offset along the outside of a rectangle, is a rounded rectangle. In the image below, black is the rectangle, green is the tool diameter, and the red line is the resulting cut path:
The reason it alternates is that every other nested rectangle flips from being “inside” to “outside”, for exactly the same reason that the lead in flips to the other side. It’s possible to not round the corners on outside offsets, but they have no effect at all on the resulting cut shape (if you specified the offset correctly), and they produce less jarring direction changes, so they’re preferable for the machine.