Your (under)exposure error of a couple stops would not cause any such graininess on Acros, it'd just cause a little shadow loss. It might be down to the developer maybe if you used print chemistry? What you've shown isn't anything like normal Acros behaviour or graininess; it should produce a grain-free print at 16x20 or so from that size negative.

My guess is that it's a grain-aliasing scan problem magnified by the huge contrast boost given to recover the probably-quite-thin negative.