When you have drawn a spline and then select to create a Mesh Spline out of it, it creates a “Triangulation” of the spline, meaning that it creates a StaticMeshActor which is not editable by the spline anymore.
This might not be the solution you want but, you can go into the selection mode and select the spline and modify it, and then go back into the Modeling mode to generate a new spline mesh.
However, this might be able to do with a spline BP in combination to PCG, but I have not tried.