I've been tuning to print at a 0.1mm layer height, the ironing turned out like this. The left side of the Z is a little textured and more shiny.
Why did this happen and how could I fix it?
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.