Landscape reverses deform to spline after Build All and randomly. Why?

So I have my landscape deforming to fit my splines in a lot of differently places but I recently realized that when I Build All or even just randomly, it will reverse that and instead of the landscape forming around the splines, it will go back and cover them up. Why is that and how could I go about fixing that? It’s not a huge problem, but it’s definitely an annoyance.