On top of the comments by
@FishyFish on the particular film/developer combo, I would add that in my experience, most scanned 400ISO films show extremely fine grain in 6x6 and above formats unless underexposed or overdeveloped or both. Underexposure and overdevelopment can, when scanning negatives, enhance grain or result in artefacts that visually approximate excessive grain
Another culprit is often oversharpening - sometimes Noritsu/Frontier scanner software as well as Silverfast etc automatically apply liberal amounts of sharpening to the scans that increase perceived grain.