I open your file and took a look at the issue that you are having. The problem is actually in the Loft2 surface feature where, if you look close to the boundary where the Fill5 surface feature is created from that area, you will see that the geometry that is created is actually different from the guide curve. I have seen this in other models and the only solution was to rethink how the design was made and typically with a boundary surface rather than a loft. I would try this approach and see how that works out. Good luck!