LinuxCNC - No straight lines in the Backplot!

Hello,

I'm quite frustrated... I try to "simulate" the carving of a simple 20mmx20mmx10mm (with 5mm rounded corners) pocket with LinuxCNC. I've produced the gcode by using g-simple:

Below what I see in the "Backplot" windiw when I run the gcode in LinuxCNC:

As you can see, at the end of every straight path the router produces a curve (!). It doesn't stop and then goes up like it should do.
:ohmy:

I'm not actually milling, I'm just observing the Stepcraft router going around with no piece on the worktable before going with the real milling.

I've double checked my Stepcraft configuration:

max axis velocity = 30 mm/s
max acceleration = 40 m/s^2

The tool has

Feed rate = 600 mm/minute (F600 instruction in the gcode)
Plunge rate = 100 mm/minute (F100 instruction in the gcode)

Homing and positioning (manually or via gcode) are perfect, the machine seems working like a charm. So... what's going on?
Obviously I could use a lower feed rate (let's say F300 or 300 mm/min), but even so I can note little curves and no straight lines in the Backplot window. With lower values I will go in pension before the milling ends...
🙁
Any clue?
I'm missing something?