Free(OnGrid()) boundary conditions produce surprising results

I'm interpolating a 3-dimensional regular surface with `scale(interpolate(Espldata, BSpline(Cubic()))` where is either `Line(OnGrid())` or `Free(OnGrid())`. I am confused by the below result, which is a 1D slice through the grid with the other two parameters being grid points. Why do the Free boundary conditions behave so poorly? My impression is that Free BCs are the same as not-a-knot (continuity in third derivative for second-last spline knot) but this is clearly not what is happening here.