https://sourceforge.net/tracker/?func=detail&aid=3049263&group_id=152762&atid=785191

data.get("items") returns selected item's id instead of a list of items

The method is: org.zkoss.zk.ui.event.SelectEvent.getSelectEvent(AuRequest request)

and the peace of code:
return new SelectEvent(request.getCommand(), comp,
AuRequests.convertToItems(desktop, (List)data.get("items")),
desktop.getComponentByUuidIfAny((String)data.get("reference")),
AuRequests.parseKeys(data));

This only happened to me when the .zul file was included with servletContext.getRequestDispatcher(path).include(request, response); Otherwise works perfectly.

Checking whether data.get("items") is a String and convert it into a single item list would be a solution.