The structured object system in Cinema 4D is very comprehensive easy to use, but it comes with a few drawbacks.
One of them is that objects are evaluated in hierarchical order. If there are dependencies between objects, this can be things confusing, as objects might be evaluated before the objects they depend on.
These dependency issues are inherent to Cinema 4D's object system, and can be the source of frustration. However, there are ways to avoid that.
The spline object is used as a Field in the Adjustment Operator's Masking tab.
However, the Adjustment Operator might be evaluated before Cinema 4D has evaluated the spline object. You might experience lagging of delayed refresh of the masking results.
Now the spline object comes before the Terrain setup. Therefore, Cinema 4D evaluates the spline object first. It is up to date when the Terrain Object (and, hence, the Adjustment Operator) is evaluated. No lagging or delayed terrain refresh will occur.