https://drafts.csswg.org/css-backgrounds-3/#shadow-shape contains this piece of text:

However, in order to create a sharper corner when the border radius is small, when the border radius is less than the spread distance, the spread distance is multiplied by the proportion 1 + (r-1)^3, where r is the ratio of the border radius to the spread distance, in calculating the corner radii of the spread shadow shape.

This seems to have originally been added in this commit: 170e753

Has this been implemented by any browser? Firefox has not implemented it, and judging by the code here and here, Chrome hasn't either.

If there has been no interest from browser vendors, it may be a good idea to remove this section again.

If you decide to keep it, I think the case where the spread amount is negative needs some clarification (and the math may need to be double-checked).