Spline Extrude generates a hard normal seam on a mesh that should be smoothly shaded when extruding a continuous spline

How to reproduce:

1. Open the attached "IN-149007" project
2. Open Assets > Scenes > SampleScene scene
3. Observe the track surface in the Scene view

Actual result: Visible hard line runs across the track surface where the shading abruptly jumps, as if two halves were cut and pushed back together instead of blending
Expected result: The surface shades as one smooth, continuous gradient across that edge, with no visible seam

Reproducible with: 2.7.1 (6000.0.0f1), 2.9.0 (6000.0.0f1, 6000.7.0a5, 6000.6.0b9)

Reproducible on: Windows 11
Not reproducible on: No other environments tested