I didn't see any problem ( even if you post a Jpg and not a PNG, becasue JPG can introduce artifact in little color gradient like you point in you image ).
The default pen use primitive splatting along a stroke method which can introduce ( very little ) anti aliasing inconsistency.
But we have introduce with the version
0.249 another brush which only rely on a single vector shape.
It's the
Free Fill default brush and the
PondPlant Fill default brush. With these brushes you have the same quality than with Illustrator ( however you can't edit again the shape when you've finished the stroking ).
You can change the Cloning type to have a line like shape like this :
.
I know it's not the same kind of brush than the default one but for the the moment it's the only workaround to have a vector like quality in Black Ink.
More brush engine will come next which perhaps will fulfill your expectation