This is something that I've come across a number of times, but it's never been a big thing for me and it sort of comes and goes, even within the same drawing, same drawing session, so I've ignored it.
Mostly the problem's been on "slivers" of contours, almost like the face isn't there even though it is. I think it's something to do with the mask not recognising a couple of individual faces of the surface object, but I don't know.
Sorry that doesn't help, but at least you're not alone.
Trevor