PrimeFaces lazy scroll datatable has repeated items after ajax update - ajax

Question

I have a datatable with lazy load and livescroll that works fine at first glance.
In the beginning the datatable loads items 0-20, when I scroll down items 20-40 are loaded and added to the datatable so that 0-40 are shown.
Problem: When I update the complete datatable (or form) via ajax the datatable is still 40 rows large but the items from 0-20 are repeated in rows 21-40.
The parameters of the LazyDataModel load() method are strange:
first: 0
pageSize: 20
items.xhtml
<p:dataTable var="_item" value="#{controller.lazyModel}"
selectionMode="single" selection="#{controller.selection}" rowKey="#{_item.id}"
scrollable="true" scrollRows="20" scrollHeight="125"
liveScroll="true" lazy="true" >
<p:commandButton value="Update" update="#form"
action="#{controller.updateSomething}" />
ItemController.java
#ViewScoped
#ManagedBean(name = "controller")
public class ItemController
{
private LazyDataModel<Item> itemLazyModel;
private Item selectedItem;
#EJB(lookup = "ejbpath...")
protected DBService dbService;
#PostConstruct
public void init()
{
itemLazyModel = new ItemLazyModel(dbService);
}
public void updateSomething()
{
// do something unrelated to the item list
}
// Getter and Setter for itemLazyModel and selectedItem
}
ItemLazyModel.java
public class ItemLazyModel extends LazyDataModel<Item> {
private DBService dbService;
private Map<String, Item> data;
public ItemLazyModel(DBService service)
{
this.dbService = dbService;
data = new LinkedHashMap<>();
setRowCount(dbService.getItemsSize());
}
#Override
public Item getRowData(String rowKey)
{
return data.get(rowKey);
}
#Override
public Object getRowKey(Item object)
{
return object.getId();
}
#Override
public List<Item> load(int first, int pageSize, String sortField, SortOrder sortOrder, Map<String, Object> filters)
{
List<Item> result = dbService.getItems(first, pageSize);
for (Item item : result)
{
data.put(item.getId(), item);
}
return result;
}}
When changing to a paginated lazy datatable everything is fine. The datatable is on the correct 2nd page after the update and shows items 21-40.
If I set the pageSize manually and use getPageSize() in the load method, I am able to return the correct items from 0-40. But the rendered datatable still has the same problem with repeating items 1-20.
Is there maybe an incompatiblity between lazy, liveScroll and ajax updates where the datatable forgets its current scroll position (like the page when paginated)?
JSF: 2.2.13 (Mojarra)
Primefaces: 6.1