Actually the behaviour that you described is expected by design. When server paging is enabled the DataSource keeps only the records from the last fetched page which is why the dataItem for all previous pages is undefined.
If you need to store some information about the records from the previous pages I suggest you to do that via data-attributes in the mark-up.
the Telerik team
Join us on our journey to create the world's most complete HTML 5 UI Framework - download Kendo UI