In general, this may occur because not all of the new items have an id, the Grid is expecting an id for every newly added item.
As for the refreshing the Grid, this can be done using the refresh method
of the Grid, and this will re-create the Grid with the current data without making a new request to the server:
As the described scenario is specific, it may require additional modification that we are overlooking at this moment. If additional assistance is needed, please send a runnable example, so I can provide a suggestion best suited for it.
Telerik by Progress