There are also some other areas where things are not meeting up with a whole lot of accuracy:





There is a gap of about 0.002 units between those edges, which is getting a bit large - it seems that the curve used to cut that sunken in area was maybe not quite aligned very tightly and that's causing enough little irregularities to cause problems.

Maybe instead of trying to sink in a cut for that area you should instead try to do a boolean union between 2 pieces instead.

- Michael