I do not think it has to do with render settings. Actually, I think it might have to do with 'traditional' banding, the fact that you are generating stept between tints (shades of color) that are so small, that sometimes there has to be a slight jump between one and the next.
In greyscale there are only 256 shades, but I do not know how many steps can be made in an RGB channel in Cararra.

If this assumption is correct, adding a little 'Turbulence' to the light cone should help, a non-egal backgrond or, the trick I use when this happens for material for print, adding a bit of noise in post.

But I might be totally wrong on this...