(Thumbnail picture unrelated but required)
Setup:
You'll want a shader that displays tangents to see what the issue is.
Instead of displaying tangents themselves, you may want to instead render the dot product between the tangent and normal to visualise how perpendicular they are.
Note that tangents are only invalid on fuselages (possibly other parts that are similarly procedural), parts like droods and wings have (mostly in the case of droods) valid vertex tangents.
Test instructions:
1. Open the game and load mod with tangent/dot shader
2. See that the tangents are invalid (do not match surface) or that the tangent and normals are not perpendicular
3. Enter build
4. See that the tangents/dot product are still invalid
5. Enter flight
6. See valid tangents or dot (dot should be black, i.e. completely perpendicular, but there may be inconsistencies from vertex interpolation in the shader)
7. Exit to designer
8. See valid tangents or dot
9. Exit to menu
10. See valid tangents or dot