Thank you for writing back.
The collapse operation in RibbonBar is performed in the ExpandableStackLayout class and each item in it that inherits from the CollapsibleElement class can be expanded or collapsed. By default, ExpandableStackLayout checks whether the sum of all sizes of the elements is larger than the available space and calls the CollapseToStep method for these elements with CollapseStep smaller than CollapseMaxSteps. There is a property of the RibbonBarGroup, which can be set to determine when the group should be collapse - see the CollapsingPriority property. The higher this is the, the higher its collapsing priority is. In your case, you can set the CollapsingPriority of the group containing your radio button to a small number to make it disappear last.
On a collapsible element as RadButtonElement, the text of it will be hidden first, then considering that the button has both LargeImage and SmallImage, the LargeImage will be hidden, displaying the SmallImage, and only then the element will be collapsed. However, this is not valid for radio buttons since leaving such with hidden text (just the small circle) will make no use for the users, thus the whole element is hidden.
I hope that you find this information useful.
the Telerik team