Dennis @ PerfectForms
You have found a random bug. If the only object in a Check Mandatory behavior object is a Radio Button Group the border will stay red. If you add a Text Input object to the Check Mandatory list the border will return to black when the behavior runs again.
The workaround I suggest is: Add a hidden text input field and place it on a different page of your form. When the Radio button group changes field data use a Set Field to place some text in the hidden text input, then use a Show behavior object and point it to your hidden text field. Finally use another Show object and point it to your submit button. The reason for these gymnastics is that the text input field must either be clicked on or shown for the Check Mandatory to acknowledge it, but you probably don’t want this hidden field showing up highlighted. That is why I suggest another Show object pointed at the submit button.
I will submit a ticket to address this bug.