Hi.
When I add a surface boundary (outer or "Show" boundary) and this boundary avoid walls, etc... then TIN lines are wrong because they try to create triangles outside the boundary, simply "hide" a portion of the triangle.
To solve this if I have 3dpoly or FL on the boundary then I have to add them as a breaklines. But Civil 3D should consider directly the boundary as a breakline of the surface to avoid this scenario because otherwise the surface will be wrong.