I have a radgrid, its populated using its need datasource event. Everything works fine.
In page prerender, i get some values & the grid rebind is called, bt the grid is nt refreshing at this time.
What you are doing should be working fine.
If your problems persist, would please open a formal support ticket and attach a small working project with the explained behavior which we can debug locally, so that we can find the cause of the problem.