That's caused by model geometry. You can clearly see how the outer perimeter does form a "c" shape. Inspect your model, I bet
that feature can be found there or there's something wrong (intersecting).
Make sure single extrusions for external perimeters are disabled, perimeter overlap isn't set too high and try
increasing the extrusion width. If that "c"shape isn't big enough to fit 2 lines / 1 perimeter, it'll be ignored:
0.4mm extrusion width:
0.55mm extrusion width:
And because you mentioned Z Brush in another thread: It is a great tool. But exports should be cleaned up before setting up a
print job. Z Brush models often have non manifold edges and other problems. And an extremely high poly count, which is useless
most of the time in FDM printing. Some of your other screenshots did show problems with your model files, flipped normals for example.