Steps to replicate the issue:

1. Enable Virtualization
2. Enable Paging
3. Enable Scrolling
4. Enable Sorting
5. Bind enough data for a few pages
6. Run the page and in the Pager click on a different page number to change the current Page
7. Click on the Column header to Sort that Column

Observe: The Grid makes a PostBack and sorts the items, CurrentPage index gets reset back to 1.