Weird. I created a table - one row, two radio buttons. I also created a button to add an instance and the new row gets new unselected radio buttons.
If you are still having difficulty, you could always just change the raw value after creating the new instance, but you would need to used the instance manager to target the correct instance to change the rawValue of.
Unselected original
One RB selected
New instance created, RB not duplicated