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.