Cannot Insert Curve into a Complex Profile

I am needing to insert a vertical curve in a tangent section of a complex profile. My first attempt to accomplish this is by using the "Profile Insert Curve" tool. I am able to add a PVI within a tangent section of the profile, but it does not allow me to specify a curve length or K value. Upon finishing the command, I am left with a PVI with no curve. An attempt to use the "Parabola Between Elements" tool and insert a curve after the fact is also ineffective, as it creates the curve, but that curve is a separate element from the complex profile I started with.

An attempt to use the "Insert Vertex" tool followed by the "Parabola Between Elements" tool leaves me with the same result.

The complex profile was originally created using the "Profile Complex by PVI" tool. The "Profile Insert Curve" tool works to insert curves on certain tangent sections of the complex profile, but not others. Any guidance as to why the tool only works on certain tangent sections would be helpful. When I have previously encountered this problem, my workaround has been to create a brand new profile using "Profile Complex by PVI" with the extra PVI I need to include. Unfortunately, the profile I am working with now is almost 6 miles long and recreating it would be a very tedious process.