When I was multiplying and manipulating a gyroid "curved-triangle" patch, some had surface normal that was the reverse of what was desired. Select patch Surface, and invoke Moi command 'Flip'.

The concepts of "inside" and "outside" were lacking for the geometry ???

- Brian

(My patches were NOT quite perfectly smooth with each other.)

Do Splines have normals???

Edit:
Google search "Normal of spline"
StackOverflow has responses.