The issue reproduces in the following situation.

• The selection mode is Extended and the selection unit is Mixed.
  
• Multiple rows are selected.
  
• Cells of the selected rows are updated.
  
• The column of the updated cells is previously sorted (before the update of the cell values).
  
• DataTable is used to populate the RadGridView control.

To work this around set the SelectionUnit property of RadGridView to FullRow or use a collection type like ObservableCollection<T> instead of a DataTable.