If I have a checkbox that is not in false state and I disable it, it loses the check mark. I was expecting that the component still shows the check mark but it does not allow to change it.
The same happens with RadioButtonList. In disabled state all radio buttons are shown empty.
Not sure about your scenario however everything works on our demos:
My mistake. I had a CSS class that made the disabled states very close to white and it interfere with the blue background color of the components, so I didn't see that they were working.