See the live Telerik Demo: http://demos.telerik.com/kendo-ui/spreadsheet/sorting-filtering
Steps to reproduce:
1. Click the filter dropdown on the "Client" column header.
2. Expand the "Filter by value."
3. In the search box, type "terr", then check "Terry Lawson".
4. Click the "Apply" button.
5. The grid refreshes with the filter applied.
6. Click the filter dropdown on the "Client" column header (again).
7. Click into the filter by value search box, and type "agne".
Notice that the checkbox-list is empty, when it should show the item "Agnes Hill".
There is no way to choose any other items and "add to the current selection" because the search will only match by what's already been filtered. This is broken behavior and doesn't match what Google Sheets or Excel does (see attached videos). It doesn't make sense to only be able to add to the current filter what has already been filtered.
After step 7, you should see the filtered options based on the source data, and be able to check/uncheck from it.
Notes: see comparison videos for more info.