Thank you for the great response, that does mirror my experience, those tangent curves want to flip/change direction.

I can't believe that all of Fusion, and many other CAM software, is built on such a shaky "sketchy" foundation. It seems that in general 2D sketch constraint models are far from solved in computer science, this seems ridiculous as such a basic and elementary problem. It should be so obvious that the tangent doesn't want to go back on itself to create zero thickness geometry.

Wonder if the 2D sketch experience is much better in Onshape, NX, Catia, etc?