You know, looking at that pic with magnification, I reckon I can see several steps in the Y axis movement, as marked below.
None in the X axis, and the circle returns to what it should be when Y gets smaller.
But the errors are small - possibly within the limits of the following error of the driver IF it is a servo system rather than a stepper.
If I didn't know any better, I would be looking at stiction at the limits of the Y axis. I would not be looking at Mach as the circle completes.
Yeah, a few dummy circles of about that diameter with a pen would be the trick.
Cheers