"Dotted" line on y axis

That suggests the tracking adjustment is incorrect, which isn’t surprising, as the machine “worked” well enough to pass whatever criteria they use for final inspection.

The belt passes over an idler pulley at the front of the machine, in a place where it can barely be seen, with provision for tension & tracking adjustments. On mine, the pulley rests in a yoke passing through two slots in the machine frame with a pair of screws pushing against the front of the frame, almost inaccessible from anywhere I can stand.

Belts tend to drift toward the side with the lowest tension, which suggests you should tighten the screw toward the center of the machine. Mine have a jam nut locking the screw in place that must be loosened before turning the screw.

Good luck on not only finding the screws, but getting in there with a wrench and a hex wrench!

The stepper drivers in my laser are set to 5000 step/rev (switch 7 On) and yours are set to 4000 (switches 5 & 7 On). That’s close enough to make little difference, but you might want to change it back.

The belts have 3 mm pitch and the pulleys have 24 teeth, so the distance/step value at 5000 step/rev will be pretty close to:

14.4 µm/step = (24 tooth/rev × 3 mm/tooth) / 5000 step/rev

Which agrees well with the Machine Settings you showed earlier at about 14.3 µm/step.

That should be high enough to prevent any visible artifacts. Setting the drivers to 10000 step/rev (at 7.2 µm/step) and seeing no change will confirm the problem lies elsewhere.