When ribbon contains too many items some tool strips are collapsed. In such case ToolStripEx.Items returns a collection of single item - drop down button of the collapsed strip, instead of real items. I was able to work around this by using method below, however I'm not sure if that will always be valid and work as intended. I think Items property should always return the original item collection.
private ToolStripItemCollection GetItemCollection(ToolStripEx strip)
if (strip == null) return null;
if (strip.State == ToolStripEx.ToolStripExState.Collapsed)