Can you select the wheel, export it as a new Rhino 8 file and share that here, please?
Caster.3dm (3.2 MB)
I the past, Maxwell had the same issue. I think it had to do with the alpha channel.
Your issue is tracked in RH-84042 Ground Plane - Shadows only, grainy edges in render . Unfortunately I can’t make the item public, since it also contains some confidential material.
I assume you do need the shadows-only ground plane and the transparent background?
yes, the object on a transparent background, shadows only, no ground plane. A shadows channel could be helpful.