It’s because your nozzle is ever so slightly too low when you’re printing. I’d adjust your Z offset by one click upwards.
Reprint the cube, and see if that resolved it. If not adjust upwards by another click.
If after a handful of times you can’t get a decent ironing pass, I’d take a look at your extrusion multiplier. Drop it down by 0.01 and recalibrate and retry doing the above ironing calibration sequence again.
It’s because your nozzle is ever so slightly too low when you’re printing. I’d adjust your Z offset by one click upwards.
Reprint the cube, and see if that resolved it. If not adjust upwards by another click.
If after a handful of times you can’t get a decent ironing pass, I’d take a look at your extrusion multiplier. Drop it down by 0.01 and recalibrate and retry doing the above ironing calibration sequence again.