Starting from an empty room, which will be my furniture stockpile at some point:
One z-level down to my workshops. Note that the empty spaces in the workshop are not smoothed: (if I loo(k) at these tiles they become smoothed in appearance afterwards)
Going down to the stone stockpile:
And back up to the workshops. Apparently, the images in the stockpile triggered a redraw of sorts:
And back up to the empty room, which now has a smoothed appearance in some tiles:
These tiles revert to rough floor if I loo(k) at them.