Instead of explaining why it might fail, I should have looked more deeply into your code, which I have done now. Simplified a little further, eliminated Extend Curve, changed Graph Mapper type to ‘Bezier’ (which has handles to shape the ends better), and it works fine, No tiny bevel at the corners.
boxx_2021Oct19a.gh (25.6 KB)
I removed some unnecessary Domain and Bounds components because the defaults (0 To 1) work fine.
Please don’t use Bifocals, it’s intrusive and annoying. ‘Sunglasses’ is far better: