When the OutlookBar is minimized during run-time change of themes the text set to item's header overlaps the minimizing toggle button.

You can work this around by creating a custom control that derives from RadOutlookBar and override its OnApplyTemplate() method. Inside the method call the protected VisualStateChanged() method.

public class CustomOutlookBar : RadOutlookBar
{
	public override void OnApplyTemplate()
	{
		base.OnApplyTemplate();
		this.ChangeVisualState(false);
	}
}

Available in the R1 2017 SP1 Release.