There is a slight grain to my eyes, but I can barely focus my eyes close enough to see it, but on grey backgrounds it's ever so slightly not as clean as on the mini-led display. I've always found OLEDS generally have a slightly less clean look than LED, when you look closely, but it's an absolute non-issue for me; my panel looks great.
Here's mine, it doesn't look like that to the eye though, the grey looks very uniform visually.