Below you will see a simple demo project illustrating my case. I am also able to get almost similar behavior using the demo site.
1) Load a RadGridView with around 100 rows of data. (Not sure at what point this becomes an issue, but we noticed it here).
2) Group by one of the columns so you get some nicely spaced groupings. I had 5 groups of 20 records each.
3) Scroll to the bottom of the RadGridView.
4) Collapse the bottom most group.
You should see some large white spaces in the grid, if there is any data left at all. On the demo site, it seems like sometimes the data will eventually show up. Sometimes not.
I am using the latest dll's - q3 build 1129.