Selecting an option button, unselects other option buttons in the same group, so set the default button to True and then one is always selected.

Change the code to

Private Sub Button1_Change()
    If Button1.value = True Then
        FillBM "Reason", "Lorem ipsum dolor sit amet, consectetur adipiscing elit."
    Else
        FillBM "Reason", "Proin sed nisl enim. Cras in nisl tempus, scelerisque mi id, vulputate arcu."
    End If
End Sub