Same thing has been in place for every version of Axure. I consider it a bug, or at least an unfortunate limitation. It would be nice if there were an additional style for “selected and disabled”
To get around this, I’ve had to build my own checkboxes and radios by scratch, as dynamic panels with one state for each checkbox state. You could probably build a two state dp, one with your standard checkbox, one with a duplicate styled as you want selected and checked to look, so that when a selected cb becomes disabled, the dp changes to State2.