The red dotted line is an edge that is not spliced, and there are two situations: 1. The edge of the gap; 2. The edge of the intersection.

These are all caused by format conversion, and the red dotted line can be avoided by changing the accuracy of stitching. The default precision of UG is 0.0254 mm, and you can change it to 0. 1 or above, as long as it can be spliced without crossing and gap, but it should be as small as possible. Too high stitching accuracy will lead to serious deformation and distortion of the surface.