On the other hand the same figure with same values and a radial gradient makes always good its job. I have modified the soft edge to 256 (what is the maximum and always render correctly).
For reference the same shape without gradient looks like this (only soft edge):
It doesn't matter if you apply gradient first or soft edge first it always have same bug:
Values for this shape samples
Soft edge = 100
By the way now we know how works the linear gradient. It makes a gradient over the contour the shape and mask it with the shape itself. The big soft edge reveals the masking operation.
It would be interesting to know how makes Moho the radial gradient...
The original shape without effects.
-G