The issue exists with RadMenuItem as well!

Workaround:

public class CustomCommandBarButton : CommandBarButton
{
    protected override void OnClick(EventArgs e)
    {
        MouseEventArgs args = e as MouseEventArgs;
        if (args.Button == System.Windows.Forms.MouseButtons.Left)
        {
            base.OnClick(e);
        }
    }

    protected override Type ThemeEffectiveType     
    { 
        get    
        { 
            return typeof(CommandBarButton);     
        }
    }
}