Extending CSGCombiner with GlobalClass doesn't show in node creation menu

I'm on the stable version of 4.3 with .NET enabled, and I can't seem to extend CSGCombiner and have it show up in the node selection menu.

I am using C#, but I've tried doing the same thing with GDScript to the same effect. Removing the tool attribute doesn't work either. The only thing that works is not extending CSGCombiner. I also noticed in GDScript that the class was exposed like a global class (that is, my LSP showed it after typing "extends").

I've done some searching and can't find anyone else with this issue, and although it could be intentional, I don't see any indication anywhere that it is. Is this a bug?

For reference, here is the start of my class definition:

[GlobalClass, Tool]
public partial class CsgStairs3D : CsgCombiner3D
{
...
}