Bug report

When the persistSelection option of the grid is disabled and several rows are selected across several pages, the selectedKeyNames() method returns all IDs instead of only the ones on the current page.

Reproduction of the problem

1. Set persistSelection to false.

2. Select several rows across several pages.

3. Call the selectedKeyNames method

4. Observe that IDs that are not on the current page are returned.

The steps can be replicated in the following live demo:

https://demos.telerik.com/kendo-ui/grid/checkbox-selection

### Environment

* **Kendo UI version:** 2021.1.224
* **jQuery version:** 1.12.4
* **Browser:** [all]