To reproduce: 
- Add grid to a blank form. 
- Add summary rows and group descriptors.
- Add rows upon a button click.
- You will notice that not all rows are visible.

Workaround: 
radGridView1.TableElement.Update(GridUINotifyAction.RowHeightChanged, null);
radGridView1.TableElement.VScrollBar.Value = radGridView1.TableElement.VScrollBar.Maximum - radGridView1.TableElement.VScrollBar.LargeChange;