As a workaround, you can add a custom Margin and Padding to the MinimizedOutlookBarItems:
<Style TargetType="outlookBarPrimitives:MinimizedOutlookBarItem">
<Setter Property="Padding" Value="5" />
<Setter Property="Margin" Value="2" />
</Style>
Where xmlns:outlookBarPrimitives="clr-namespace:Telerik.Windows.Controls.OutlookBar;assembly=Telerik.Windows.Controls.Navigation".
Hi, The fix for the issue will be available in next LIB version (2018.3.924). It will also be included in our official version – R3 2018 SP1, scheduled for the mid of October, 2018. Best regards, Ralitsa Kumanova