When the RadRichTextBox is focused via clicking on it or via RadRichTextBox.Focus() the visual feedback is not diplayed.

Workaround: create a style trigger for that (for example for the Office2016 theme):

<Style TargetType="{x:Type telerik:RadRichTextBox}" BasedOn="{StaticResource RadRichTextBoxStyle}">
    <Style.Triggers>
        <Trigger Property="IsKeyboardFocusWithin" Value="True">
            <Setter Property="BorderBrush" Value="{telerik:Office2016Resource ResourceKey=AccentFocusedBrush}" />
            <Setter Property="BorderThickness" Value="1" />
        </Trigger>
    </Style.Triggers>
</Style>