You are right. I get the same here. Not on the Marker – only on the Pencil. It does it no matter the opacity setting. It is all about the size being a the very minimum, i.e. 0.5px – 0.7px or so.
As you know the pencil has paper texture on it to make it a bit rough. The roughness seems to become more evident at this very small size. It is kind of on purpose, but maybe it is too much.
Thanks for your observation! I’ll see if we can tweak it just a tad. 🙂