Put a hidden radio button in the group (set its visibility to false) and set the value of the RadioButtonGroup to its value.
Not sure I know how to do that or know what you mean by hidden radio button?
(Keep in mind I'm not an Adobe expert)
I found something that describes my issue better than I did--
There are two or more radio buttons within a single group which have the same value. Clicking on one causes the dot to appear briefly and then move to the other radio button with the same value.
When creating a form, how do I make it so a group of radio buttons don't have the same value?
Did using the component properties to set them to different values not work for you?