I had the same problem when i was using radio button for the first time in my application.
I did some changes in the cell properties(changing the orientation, label format, etc), none of them related to the default radio button selection and then after it started displaying without a default value, i don't know how that fixed it but that's how it happened for me.
When exactly do you see the value get selected? Is it when a screen is first loaded or upon some other action? When you see this, is the related property on the Clipboard updated? Is it always the same radio button that's selected (same relative position or same value)? Finally, can you check (with Tracer) if there are any activities or data transforms that may be initializing this property to the selected value?