FYI:
AFAIK -- List schemes are only saved upon the first time they are created and saved.
After the next time you open the plan, you can re-load the scheme and make changes, but the changes then are only held in memory, you can not then save the scheme and once the program is closed, those changes are lost. After the the initial save, permanent changes can only be made offline by manually changing the XML file. As a alternate, you could create a completely new scheme with the new features needed, name and save the scheme (file), and just switch to it when you need to.
There is a old 20 year paper which explains the file format and how to change it offline.
I don't know if this is a bug or a feature,but it is the reason few use list schemes.
You may want to take a look at the new Python feature, which can parallel those old list features -- SOMEWHAT
Gerry
Windows 11 - Visual Studio 2022; ArchiCAD 27