Are the two classes in the same file, or two different files? If they are in the same file, that could be causing the issue. In GDScript, Godot treats each file as a single class, with classes defined in that class as a sub-class, and I think sub-classes exports do not work in GDScript. This may also be the case with C#, where Godot thinks it is a subclass and therefore doesn't show the exported property.